New docs gallery profile example

[ukmo-huw-lewis]

Fixes #2416

Contribution checklist

Aim to have all relevant checks ticked off before merging. See the developer's guide for more detail.

• Documentation has been updated to reflect change.
• New code has tests, and affected old tests have been updated.
• All tests and CI checks pass.
• Ensured the pull request title is descriptive.
• Attributed any Generative AI, such as GitHub Copilot, used in this PR.
• Marked the PR as ready to review.

MS CoPilot used in initial development of gallery within #2353

[github-actions]

Total coverage: 93% (HTML report)

Name                                                              Stmts   Miss Branch BrPart  Cover
---------------------------------------------------------------------------------------------------
src/CSET/__init__.py                                                105      0     14      0   100%
src/CSET/_common.py                                                 156      0     54      0   100%
src/CSET/cset_workflow/app/fetch_fcst/bin/fetch_data.py             117     28     26      0    78%
src/CSET/cset_workflow/app/fetch_nimrod/bin/fetch_nimrod.py          81      8     28     11    83%
src/CSET/cset_workflow/app/finish_website/bin/finish_website.py      79      2      8      2    95%
src/CSET/cset_workflow/app/parbake_recipes/bin/parbake.py            29      0      8      0   100%
src/CSET/cset_workflow/app/send_email/bin/send_email.py              25      0      4      0   100%
src/CSET/cset_workflow/lib/python/jinja_utils.py                     17      0      6      0   100%
src/CSET/extract_workflow.py                                        103      1     26      1    98%
src/CSET/graph.py                                                    44      0     14      0   100%
src/CSET/operators/__init__.py                                       89      0     26      0   100%
src/CSET/operators/_atmospheric_constants.py                          9      0      0      0   100%
src/CSET/operators/_colormaps.py                                    249      3     72      4    98%
src/CSET/operators/_stash_to_lfric.py                                 3      0      0      0   100%
src/CSET/operators/_utils.py                                        190      8     72      6    95%
src/CSET/operators/ageofair.py                                      142      7     64      5    94%
src/CSET/operators/aggregate.py                                      77      1     22      1    98%
src/CSET/operators/aviation.py                                       61      0     18      0   100%
src/CSET/operators/collapse.py                                      155      8     72      3    93%
src/CSET/operators/constraints.py                                   113      7     50      3    93%
src/CSET/operators/convection.py                                     38      4     10      2    88%
src/CSET/operators/ensembles.py                                      27      0     14      0   100%
src/CSET/operators/feature.py                                        44      0     10      0   100%
src/CSET/operators/filters.py                                        67      2     30      0    98%
src/CSET/operators/fluxes.py                                         41      0     10      0   100%
src/CSET/operators/humidity.py                                      135      0     52      0   100%
src/CSET/operators/imageprocessing.py                                57      0     16      0   100%
src/CSET/operators/mesoscale.py                                      18      0      2      0   100%
src/CSET/operators/misc.py                                          172      1     72      3    98%
src/CSET/operators/plot.py                                         1117    155    412     71    83%
src/CSET/operators/power_spectrum.py                                 98      3     30      3    95%
src/CSET/operators/precipitation.py                                 204      2     92      2    99%
src/CSET/operators/pressure.py                                       41      0     12      0   100%
src/CSET/operators/read.py                                          439     22    190     17    94%
src/CSET/operators/regrid.py                                        147      1     70      3    98%
src/CSET/operators/scoreswrappers.py                                323     23    106      2    93%
src/CSET/operators/temperature.py                                   121      0     32      0   100%
src/CSET/operators/transect.py                                       63      0     24      0   100%
src/CSET/operators/wind.py                                           46      3     10      2    91%
src/CSET/operators/write.py                                          15      0      6      0   100%
src/CSET/recipes/__init__.py                                        104      0     28      0   100%
src/CSET/sample_data/__init__.py                                      0      0      0      0   100%
---------------------------------------------------------------------------------------------------
TOTAL                                                              5161    289   1812    141    93%

[ukmo-huw-lewis]

Vertical profile example builds on timeseries plot example previously reviewed under #2413 and now merged to main.

Changes here

• introduce new line examples directory with GALLERY_HEADER.rst file
• introduce new vertical profile line plot gallery doc page plot_line_vertical.py
• add new sample_data files profile_air_temperature_1.nc and (copy of test_data transect_test_umpl.nc, but extracting first timestep and 2 lat/lon points only) and profile_air_temperature_2.nc (copy of profile_air_temperature_1.nc with iris np.random.normal noise added).
• Updated default _colorbar_definition.json file to set wider default xmin and xmax for temperature_on_pressure_levels variable definition.

Example output in built html accessible in: plot_line_vertical.html

Marking as ready for review.