DOC: start v1.6.0 release notes
xref #45198
Guess these need updating 

```
2022-08-23T18:04:09.3434488Z /home/runner/work/pandas/pandas/doc/source/whatsnew/index.rst:30: WARNING: toctree contains reference to nonexisting document 'whatsnew/v1.6.0'
2022-08-23T18:04:09.3435191Z /home/runner/work/pandas/pandas/doc/source/whatsnew/v.1.6.0.rst:225: WARNING: Explicit markup ends without a blank line; unexpected unindent.
2022-08-23T18:04:09.3435768Z looking for now-outdated files... none found
2022-08-23T18:04:09.5629983Z pickling environment... done
2022-08-23T18:04:09.5644924Z /home/runner/work/pandas/pandas/doc/source/whatsnew/v.1.6.0.rst: WARNING: document isn't included in any toctree
``` > Guess these need updating

same problem in https://github.com/pandas-dev/pandas/pull/45198, needs a blank line, see commits in #45198