docs: bring back ALTSVC.md and HSTS.md

This partly reverts 0e06603b2318356ba78b2f

These file formats are not properly documented elsewhere, plus the
website uses these files to

docs: bring back ALTSVC.md and HSTS.md

This partly reverts 0e06603b2318356ba78b2f

These file formats are not properly documented elsewhere, plus the
website uses these files to populate the documentation pages to which
users end up via the URLs that are mentioned within the alt-svc and hsts
files.

Fixes #15705
Reported-by: Jeffrey Bosboom
Closes #15706

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