Move data about sidebars and headers from HTML to YAML

[bershanskiy]

MDN has a lot of macros which insert headers and sidebars, including:

• Draft
• SeeCompatTable
• sidebars like AddonSidebar, ToolsSidebar, APIRef, etc.
• securecontext_header
• AvailableInWorkers

I think it's possible to move these declarations to YAML header. Then later we could even cross-check some of them with BCD.

Examples

Macro	YAML equivalent	Notes
Draft	draft: true	Like in Hugo frontmatter, if draft is omitted or false document is not draft. Any value besides true and false is invalid.
non-standard_header	standard_track: false	Like BCD's standard_track
deprecated_header	deprecated: true	Like BCD's deprecated
SeeCompatTable	experimental: true	Like BCD's experimental
AddonSidebar	sidebar: addon
ToolsSidebar	sidebar: tool
APIRef("_string_")	sidebar: "_string_"	Complete list of permitted strings inferred from content.

[peterbe]

We could start with just setting the sidebar in the front-matter. Other KS macros like non-standard_header feels more OK to leave to another day. The reason is that the sidebars are treated very differently in Yari, but things like {{SeeCompatTable}} are just HTML as far as Yari is concerned.

What's exciting to me is the potential to rewrite the sidebars. Instead of generating the sidebars from a KS macro (with full HTML) we can instead write a new piece of Node code that traverses the tree of files and extracts all the URLs + titles and then groups them into a plain array.