Fixes to change policy

https://github.com/apache/www-site/blob/ec40482d32a24ef4c1b451ecb4fa32d7fdcbfe3a/README.md?plain=1#L3

Note that some of the content of the website is under the purview of groups other than M&P.
For example files under legal must be approved by the legal team.
Also some parts by trademarks etc.
See https://github.com/apache/www-site/blob/ec40482d32a24ef4c1b451ecb4fa32d7fdcbfe3a/.asf.yaml#L16 for a list of paths and interested parties.

Also note that historically, any member was allowed (even encouraged) to make fixes to the website, so changes to policy need to be made really obvious. I think it would help to have a separate section on change approval. The change should also be announced to members@ once it has been fully documented.

Start with README.md at the linked section and .asf.yaml around line 16 to identify paths requiring approval from legal, trademarks, and other groups. Update the change policy with a clearly visible approval section, then ensure the completed policy is announced to members@ as requested.

Written by the indexing model from the issue text.