sysadm: Move keycloak documentation to sysadm instance

This also:

• installs redirection from old location to the new one.
• install the intended audience on respective documentations.

example (non exhaustive)

Related to swh/infra/sysadm-environment#3154 (closed)

Test Plan

compilation instructions happy:

make -C docs html
make -C sysadm html

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Migrated from D6509 (view on Phabricator)