Add python_version input to doc build workflows

[FabienDanieau]

What

Adds an optional python_version input to both reusable build workflows (build_main_documentation.yml and build_pr_documentation.yml). When set, the build venv is created with that Python:

uv venv ${{ inputs.python_version && format('--python {0}', inputs.python_version) || '' }}

Unset by default → bare uv venv, i.e. no behavior change for any existing caller.

Why

The build venv is currently created with a bare uv venv, which picks the runner's system Python — 3.10 on ubuntu-22.04. A documented package that requires a newer Python then can't be installed, and the build fails during "Setup environment":

× No solution found when resolving dependencies:
╰─▶ Because the current Python version (3.10.12) does not satisfy Python>=3.11
    and reachy-mini==1.9.0 depends on Python>=3.11 ...

There's no way for a caller to select the interpreter today: pre_command runs after the venv is created, and a reusable workflow can't inherit env like UV_PYTHON from the caller. This surfaced building the pollen-robotics/reachy_mini docs (requires-python = ">=3.11").

uv provisions the requested version automatically (the job already exports UV_PYTHON_INSTALL_DIR), so callers just pass e.g. python_version: "3.11".