doc/install: Linkify mention of ceph.conf and use ref for links

doc/install: Linkify mention of ceph.conf and use ref for links

Linkify first mention of config file to ceph.conf docs in:
- install-storage-cluster.rst
- manual-deployment.rst
- manual-freebsd-deployment.rst

Use ref instead of an external link in:
- clone-source.rst
- get-packages.rst
- index_manual.rst
- install-storage-cluster.rst
- manual-deploymen.rst
- manual-freebsd-deployment.rst
Only where a label already exists at the destination.
Delete the old link definition if one was used previously.
That should be about all external links in install/ that can use
existing labels for ref.

Fix an instance of "the a" into just "a" that is consistent with other
similar mentions in manual-freebsd-deployment.rst.

Signed-off-by: Ville Ojamo <14869000+bluikko@users.noreply.github.com>