website: Add navigation between the documents of a release

Every release publishes a set of sibling documents (announcement, signed
checksums, upgrading or installation instructions, hardware notes, release
notes, errata, readme and schedule), but they are only listed together on
the release index page. Once inside one of them there is no way to reach
the others without editing the URL, which is inconvenient for the documents
most often read before an update, such as the errata and the upgrading
instructions.

Add a discreet list of the other documents of the same release under the
page title. The list holds the documents the release index page links to,
so releases shipping a different set of documents are handled without any
per-release maintenance, and the documents of an upcoming release stay
hidden until the index publishes them.

Only releases whose directory has its own _index.adoc are covered, which is
10.2R and newer.

The new i18n keys are added to ru.toml and es.toml in English, for the
translators to pick up.

Reviewed by: cperciva
Pull Request: https://ron-dev.freebsd.org/FreeBSD/doc/pulls/3