doc/start: remove "intro.rst"

Remove "start/intro.rst", which has been renamed "start/index.rst" in
order to follow the conventions followed elsewhere in the documentation.

Follows ceph#57900.

Signed-off-by: Zac Dover <[email protected]>

Author: zdover23

doc/start/index.rst

@@ -97,7 +97,3 @@ recover dynamically.
          get-involved
          documenting-ceph
 
-.. toctree::
-   :maxdepth: 2
-
-   intro