[FET][Docs] Updated FET docs and added 'components' root files

Updated the docs of the Feature Engagement Tracker and added the
`components` subfolders` root files to be included in the build config
so that they are viewable/modifiable directly from Xcode.

Fixes: 1406916
Change-Id: I77e6a9f69eca330713d43b12110749762ca0a489
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4162676
Reviewed-by: Sylvain Defresne <sdefresne@chromium.org>
Commit-Queue: Nicolas MacBeth <nicolasmacbeth@google.com>
Reviewed-by: Tommy Nyquist <nyquist@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1092431}