Fix some documentation build warnings.

Now that the docs are split in different files, we need to add myst-parser
specific markups for cross-reference links.

After this patch, there are still 2 problems reported when building the docs:

- the snippet in docs/hint_details.md creating hints_func errors out as the "$"
  character is incompatible with SQL formatting.  Leave it as-is, as the result
  is that sql highlighting is only skipped for this snippet, which would be the
  same if we explicitly removed the sql tag, but we still get a warning to
  remind us about the problem if we want to eventually find a solution.
- there's a link to some "#restrictions" paragraph in docs/hint_table.md, but
  as far as I can see this link has always been dead so I'm not sure what to do
  of it.

Pull request #125, fixes #123

Backpatch-through: 14

docs/hint_list.md

@@ -1,3 +1,4 @@
+(hints-list)=
 # Hints list
 
 The available hints are listed below.

docs/hint_table.md

@@ -156,6 +156,7 @@ postgres-# SELECT * FROM table1 t1 WHERE key = 'value';
 ...
 ```
 
+(guc-parameters-for-pg_hint_plan)=
 ## GUC parameters for `pg_hint_plan`
 
 GUC parameters below affect the behavior of `pg_hint_plan`.