internal/postgres: ensure doc HTML is valid UTF-8
internal/postgres: ensure doc HTML is valid UTF-8

We've seen documentation that is not valid UTF-8
(github.com/xuender/oil/math). That results in a DB error when written
to a TEXT column.  Make the documentation valid before writing to the
DB.

Change-Id: Ib2ccdd1dfb3b66af9d444b81216f561a8d906d8a
Reviewed-on: https://team-review.git.corp.google.com/c/golang/discovery/+/757995
Reviewed-by: Julie Qiu <e6cb64861edcc38971590ff27c111e0e672de045@google.com>
CI-Result: Cloud Build <ea5729d4cf3392b9d382b6ca272c65aa27554f44@system.gserviceaccount.com>

golang/pkgsite,golang/pkgsite,golang/pkgsite,golang/pkgsite