DOP-3424: "On this page" subnav incorrectly displays headings for all language tabs

[mayaraman19]

Ticket

DOP-3424

Notes

Creates a selector_ids object that reflects the AST in terms of the order of tabs/method selectors. This object looks something like this:

method-option: abc
children: {
     tab: def
     children: {
          tab: efg
     }
}

This is used by snooty (see PR) to determine correct headings to show.

README updates

• • This PR introduces changes that should be reflected in the README.md and/or HACKING.md, and I have made those updates.
• • This PR does not introduce changes that should be reflected in the README.md and/or HACKING.md

[seungpark]

taking a look at the frontend PR! i think it'll help me understand the usage of page.ast.options['headings'][].selector_ids