Add placeholders for dataset overviews

[znichollscr]

Description

Checklist

Please confirm that this pull request has done the following:

• [x] Documentation added (where applicable)
• [x] Changelog item added to changelog/

[znichollscr]

@durack1 @vnaik60 what do you reckon?

[znichollscr]

Relevant pages for doc reviews:

• https://input4mips-controlled-vocabularies-cvs--146.org.readthedocs.build/en/146/usage-data-user/
• https://input4mips-controlled-vocabularies-cvs--146.org.readthedocs.build/en/146/dataset-overviews/

[durack1]

Couldn't hurt to point out that we also have the CMIP6-era datasets available alongside the CMIP6Plus-era datasets.. Makes me think about how relevant it is to provide the historical context of these data and their evolution

[znichollscr]

Superseded by #150 (this can stay here for reference/to help us with drafting future info)