arrow

GH-35906: [Docs] Enable building the documentation without having pyarrow installed (#35907)

### Rationale for this change

Ease the process of building the documentation for dev purposes.

### What changes are included in this PR?

`conf.py` is updated in a way to permit having pyarrow not installed (from source or as a binary).
In case pyarrow is not available:
- `docs/source/python` folder will be excluded from the build of the documentation
-  version of the documentation will be set to `'0.0.0-local-docs-build'`

I have tested the changes for cases when:
- pyarrow was built from source
- without pyarrow
- pyarrow was installed from PyPI, version 12.0.0

with building all of the docs and only format/developers sections.
* Closes: #35906

Authored-by: AlenkaF <frim.alenka@gmail.com>
Signed-off-by: AlenkaF <frim.alenka@gmail.com>