Make the Tails documentation searchable offline

Originally created by @mercedes508 on #6569 (Redmine)

In the offline documentation, when typing keywords on the search field, the results are not results from the offline doc and thus are irrelevant.

The results are taken from the documentation indexed by the GNOME help system, that does not include Tails documentation.

How hard would it be to have our documentation registered and indexed too?

Edited May 15, 2020 by mercedes508

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