Updates templates/zerver/meta_tags.html to add a rel-canonical
link if REL_CANONICAL_LINK is in the template context dict.
We add REL_CANONICAL_LINK to the documentation context for the
API and integrations documentation pages in all cases. The link
will be self-referencing for the main zulip.com site.
For policies documentation pages, we add REL_CANONICAL_LINK to
the context only when settings.CORPORATE_ENABLED is true, so
that self-hosted servers' policies documentation do not have a
rel-canonical link set.
Part of #35110.
6fb19fdd2d