Add a release page

Summary:
This patch introduces a Releases page at /releases/. It includes a
prominent hero for downloading the latest version, release notes for the latest
version available on the page, and helpful links for other past releases.

While the page is ready for use immediately (and replaces links to Binaries
on the homepage and nav bar), some more iteration will need to be done to fit
in other download links such as the Ubuntu PPA, AUR, etc. Only once these are
all included can the release page replace more links and become the central
location for all things release-related.

Note that this patch makes a subtle change to fetch_markdown_files.sh in order
to allow embedding markdown files into pages. The usual markdown-converted-to-HTML
pages have been renamed to *.page.md since they include frontmatter and are
eventually rendered to include navigation elements among other things. There is no
change in behavior for these files.

Depends on D7617

Test Plan:

./scripts/fetch_markdown_files.sh
make serve

Navigate to http://127.0.0.1:4000/releases/

Verify markdown-converted pages still work as expected: http://127.0.0.1:4000/DISCLOSURE_POLICY.html

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7615