For each wasm feature flag, this adds a link to the proposal repository,
and states who is the v8-side owner of the implementation.
For stated and shipped features, it also sais since which v8 version
this is the case.
It also adds some documentation about the meaning of the three categories
(experimental, stages, shipped).

R=mstarzinger@chromium.org, adamk@chromium.org

No-Try: true
Change-Id: Idbb26eb9e68c7a81aed0e1c1b1d12d5de40d7c2e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1954395
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Andreas Haas <ahaas@chromium.org>
Reviewed-by: Adam Klein <adamk@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65378}