docs: add RFC links to FEATURES database in compatibility_hints.py

All entries in the FEATURES dict that reference RFCs in their descriptions
now also have a `links` field with canonical IETF datatracker URLs.  All
section references have been verified against the actual RFCs.

Also fixes one incorrect reference: the description for `create-calendar`
previously mentioned "RFC4791 section 8.5.2" as the place where MKCOL
is mentioned for calendar creation (section 8.5.2 is actually about
external attachments).  The correct references are RFC4791 section 5.3.1
(MKCALENDAR) and RFC5689 (extended MKCOL).

Prompt: In caldav/compatibility_hints.py, in the FEATURES database, there are quite many free-text references to the RFCs.  1) Download and verify that all RFC references are correct, and 2) the RFC references should not be in the free-text description, but in the link field, with canonical URLs being used.

Followup-prompt: I've changed my mind, keep the RFC references in the
free-text description, but add the links.

Followup-prompt: Don't include the link to RFC4791 in the
delete-calendar feature

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: tobixen