prepare to introduce intra-page navigation aids

[tayloraswift]

per discussion here we should have a ‘table of contents’ sidebar feature similar to DocC, for jumping to the heading on the current page.

my gut feeling is this is something we want to implement on the client-side, via javascript. for that to work, we need to get a grip on the anchored headings we generate on the server-side for each page.

• [ ] add sanitizer to SSGC, to prevent it from allowing articles to contain more than one heading with the same text
• [x] develop a mangling scheme for the extension blocks, so that we can link to the Conformances subsections inside of them. this is likely the hardest part, since no such identity exists within the Swift compiler, so we will have to invent one
• [x] emit heading anchors for those Conformances subsections

[tayloraswift]

once #269 is complete, we should have an intrapage sidebar that supports jumping to named headings, although no generated extension blocks yet (due to lack of supported mangling scheme)

[tayloraswift]

274 adds the extensions to the navigator as well