DOC: Remove absolute urls from the docs
Looks like we've got some absolute urls (including the domain) in the docs, like https://pandas.pydata.org/pandas-docs/stable/user_guide/timeseries.html#offset-aliases (see here: https://github.com/pandas-dev/pandas/blob/master/pandas/core/indexes/datetimes.py#L178 ).

Sphinx (the tool we use to build the docs) provides a way to create those automatically, with the `:ref:` directive.

See for example how to automatically generate a link [here](https://github.com/pandas-dev/pandas/blame/master/doc/source/user_guide/computation.rst#L113):
```
:ref:`caveats <computation.covariance.caveats>`
```
Which will point to the label defined [here](https://github.com/pandas-dev/pandas/blame/master/doc/source/user_guide/computation.rst#L51):
```
.. _computation.covariance.caveats:
```

To avoid having new cases in the future after fixing the existing ones, we should add a check in `ci/code_checks.sh` to validate that the pattern `pandas.pydata.org` doesn't exist in the repository (see other examples of patterns we don't want to find in `ci/code_checks.sh` that use the `invgrep` function). If there is a better patter than `pandas.pydata.org` feel free to use it.