sitegen += section links and xrefs

xmarks, xrefs:
.html files can have xmark anchors, like htmls anchor, except can be referenced from other pages.
xref is like href, but references xmarks

"pilcrow" section links: when mouse hovers over a heading, show a clickable link to that section. handy for when you want to answer a question w/a link to place in the docs.

fix to "breadcrumb" navigation: just show "ancestors", not also "older siblings"

added the start of some docs-for-docs

Testing Done:
https://travis-ci.org/pantsbuild/pants/builds/37849182

Reviewed at https://rbcommons.com/s/twitter/r/1154/
Sync of https://github.com/pantsbuild/pants/commit/ddb3b74eb614ac2fd8769735f4699001957937bd