Allow slash in config

[datamel]

This PR is a possible solution to the issue identified in Symlink Dirs PR, where, adding config item share/cycle to enable users to configure this directory for symlinking, resulted in an error when building the docs. This was because the / is a special character.

I have built the docs (with the cylc/cylc-flow#3884) - checking that the share/cycle no longer results in a doc build fail.

[oliver-sanders]

Nice job for getting your head around Sphinx domains!

Here's a quick functional test:

diff --git a/cylc/sphinx_ext/cylc_lang/__init__.py b/cylc/sphinx_ext/cylc_lang/__init__.py
index 8a7a978..d58272c 100644
--- a/cylc/sphinx_ext/cylc_lang/__init__.py
+++ b/cylc/sphinx_ext/cylc_lang/__init__.py
@@ -88,7 +88,7 @@ Sphinx domain for ``cylc`` configurations.
 
         .. cylc:setting:: pub
 
-           a setting called ``pub``
+           A setting called ``pub``
 
            .. cylc:value:: integer
 
@@ -103,6 +103,10 @@ Sphinx domain for ``cylc`` configurations.
 
               an iso8601 duration.
 
+        .. cylc:setting:: a/b/c
+
+           A setting with a ``/`` in the name.
+
 
 Auto Documenters
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[datamel]

I had a play around with the regex here: https://regex101.com/r/PR0YQc/6
In case it is of any use to reviewers - I find regexs hard to interpret without examples!