Delete files associated with old documentation

[hayleycd]

I opened #1216 to remove old documentation from the /docs folder.

However, there are other associated files that should be removed once the redirects are live.

These files include but are not limited to:

1. gcp/appengine/docs/index.html
2. gcp/appengine/frontend3/src/templates/faq.html

And there are other files that may need to be edited.

In summary:

Remove files that are no longer needed because of the doc migration, and edit files as required to reflect the new docs.

[hayleycd]

Partially addressed by #1218

[hayleycd]

Partially addressed by #1237

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