new: doc: Prepare documentation for BIND 9.21.21

I asked Claude to prepare the "Tweak and reword release notes" (663dba18f3015aefe178ba8b4790c7180f943c74) commit with the following guidance:
> add RST markup to @doc/notes/notes-9.21.21.rst. possible RST markups are to be found in @doc/arm/. if in doubt look at previous release notes in @doc/notes/. while at it, fix grammar and make sure the text is aligned to max 72 characters.

It did better that I'd do.

Merge branch 'mnowak/prepare-documentation-for-bind-9.21.21' into 'v9.21.21-release'

See merge request isc-private/bind9!970