DOC: Fixing EX01 - Added examples
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).

Towards https://github.com/pandas-dev/pandas/issues/37875

Notes:
On the web there are only 3 custom rolling windows: `BaseIndexer`, `FixedForwardWindowIndexer` and `VariableOffsetWindowIndexer`. But there are more of them in the [code](https://github.com/pandas-dev/pandas/blob/main/pandas/core/indexers/objects.py). Like `ExpandingIndexer` etc.. 

~~Added `# doctest: +SKIP` to the last line of `BaseIndexer` because `validate_docstrings.py` was complaining.~~

I need to do \preview here as I was having a lot of issues with the html file made with Sphinx.

I'll open a separate PR to correct the User Guide on `BaseIndexer`.

/preview No preview found for PR #54254. Did the docs build complete? /preview Website preview of this PR available at: https://pandas.pydata.org/preview/54254/ It looks like failing tests are unrelated Thanks @DeaMariaLeon 