Do not cache HTML of pages that are expected to change

Summary:
By default, we should assume HTML pages are going to have information updated
regularly (most notably, new release versions), especially as we apply more automation.
This fixes an issue where a user who recently looked at the homepage or releases page
will see the old version when revisiting the site after a new release becomes available.

CSS/JS and other assets are unaffected by this change.

Pages under /doc/ are not affected by this change and may be cached, since many of these
pages are versioned and unlikely to change.

Test Plan:

docker build -t bitcoin-abc-org .
docker run -it -p 8080:80 bitcoin-abc-org

Verify Cache-Control: no-cache header for the following pages:

• /
• /releases/

Verify Cache-Control is not set for the following pages (note that in prod, a default
cache duration will be applied):

• /doc/
• /doc/release-notes/release-notes-0.22.3.html

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D7770