Document anchor usage

[jo-mueller]

💯 for improving anchor usage. We likely want to document suggestions on the anchor naming strategy.

Originally posted by @joshmoore in #567

As pointed out, the anchor usage by @lubianat in #567 makes the cross-links on this page much more stable long-term, because the link will not break if anchor or reference move around. Adding a short note for this in the contrib section would be nice.

The anchor system is referenced here, but it doesn't only apply to the spec with #567 anymore.

[jo-mueller]

To document what we are already doing in terms of conventions:

• RFCs use rfcs:rfcX:comment/review/etc
• Spec documents have (ngff-spec:spec:0.X) at the top to allow themselves to be referenced cleanly, but the rest of the anchors in the spec document are document-internal. It would be a pain to have to update too many anchors on a version bump otherwise.