sensors: Update Blink's README.md.

- Adjust the Markdown formatting, some links were not generated
  properly.
- Include more sensor types in the list of sensors implemented in Blink.
- Stop referring to the outdated design doc on Google Docs, point to the
  README.md in //services instead.
- Point to the right location where the web tests currently reside.
- Mention the runtime flag that some APIs depend on.

Change-Id: I8086e3b9d32b20fdc2a8801b610c3e6a7a8dc2ca
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4555458


Commit-Queue: Reilly Grant <reillyg@chromium.org>
Reviewed-by: Reilly Grant <reillyg@chromium.org>
Auto-Submit: Raphael Kubo Da Costa <raphael.kubo.da.costa@intel.com>
Cr-Commit-Position: refs/heads/main@{#1148139}