Policy: Heading rule style

[chorman0773]

Currently rule ids for headings have to be specified using a separate rule id, either before or after the heading. This has two primary issues:

• The rule will be formatted separately from the header, and the rule style makes it seem like there should be attached content after (rather than a heading or another rule)
• The Heading will be assigned a separate anchor id rather than the rule id.

I'd like for us to support rule-id's in headings, either using the standard cmark anchor id, or an extension for mdbook-spec.

@rustbot label +C-meta-policy

[chorman0773]

@rustbot label +C-meta-formatting