docs: fill in own_data user guide page (#312)

[1abbasia]

docs/user_guide/own_data.rst has been a title-only stub since it was created in February 2022. This fills it in.

The page now lists the minimum column set with units, and walks through loading a CSV whose columns don't match — renaming, and converting the timestamp to a timezone-aware datetime.

Issue #312 describes a user with trajectory data in a CSV who couldn't work out from the docs how to load it. What makes this worth documenting is that Flight and Traffic wrap a DataFrame without validating or renaming anything, so mismatched column names don't fail at construction — they fail later, in whichever method expected the column. That's a hard first experience to debug, and loading your own data is one of the first things a new user tries.

I used .. code-block:: python rather than .. jupyter-execute:: as other user guide pages do, because I couldn't build the docs locally to verify a runnable block would pass. Happy to convert it if you'd prefer consistency.

First contribution here. I didn't find a CONTRIBUTING file, so I followed the conventions in the existing docs/user_guide/ pages — let me know if you'd like it structured differently.

Closes #312

[xoolive]

Thank you, that's very nice! I will keep it open for the moment, because I am in the middle of transferring the documentation to a new platform (branch prepare-v3) feel free to open a PR to that branch if you want, or I can just do it as well no worry!