DOC: Use a standard header for all rst files
When we build the documentation from the `.rst` files in `doc/source/`, we use a variable `common_imports` that is added as context for sphinx, and that is used in the `whatsnew` files.

But more or less the same imports and options that are used in the `whatsnew` files, are used in most `.rst` files.

What would be useful is:
- Rename `common_imports` to `header`, as this will be used for other things in the future (like defining `tocdepth`)
- Add it at the beginning of all the `.rst` files in the same way as it's done for the `whatsnew` files
- Remove the equivalent code already at the beginning of the files
- If there is anything that would be useful to add to the content of `common_imports` (because it's used in many files), add it

This can have implications on the `flake8-rst` validation, as I assume the content is validated before rendering, and the imports will be missing during the validation.

CC: @TomAugspurger @FHaase 