[vm/docs] Improve VM docs tooling.

Main change is around removing our custom syntax which allows
to read markdown files directly on GitHub:

* instead of using custom link format `@{xref}` we start
  using normal links. For example, [`dart::ThreadPool`][] is
  understood as a ref to `dart::ThreadPool` class declaration.
  `build.py` script injects an xref section at the end of
  each markdown file.
* similarly we don't use custom syntax for admonitions, but
  instead use blockquotes. `build.py` detects block quotes
  which start with a marker like `**Note**` and renders
  then in a custom way.

This CL also drops dependency on cquery and instead rewrites
indexing in pure Python via libclang.

Change-Id: I0b47ec93f632de89627a3c682d511c8b86c58430
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/280262
Reviewed-by: Martin Kustermann <kustermann@google.com>
Commit-Queue: Slava Egorov <vegorov@google.com>