Docs: Add a "seealso" section to the module docs (#45949)

* Docs: Add a separate  "seealso" section to the module docs
to list related modules and/or related references. This clears up the notes
section for things that are actual notes.

So you can add a section in your module documentation and four types of
references are possible.

    seealso:

    # Reference by module name
    - module: aci_tenant

    # Reference by module name, including description
    - module: aci_tenant
      description: ACI module to create tenants on a Cisco ACI fabric.

    # Reference by rST documentation anchor
    - ref: aci_guide
      description: Detailed information on how to manage your ACI infrastructure using Ansible.

    # Reference by Internet resource
    - name: APIC Management Information Model reference
      description: Complete reference of the APIC object model.
      link: https://developer.cisco.com/docs/apic-mim-ref/

This PR also includes:

- Implements ansible-doc support
- Implements schema support for the seealso options
- Updates to the development documentation
- Rename filter convert_symbols_to_format to rst_ify, cfr the existing html_ify and tty_ify filters
  - This makes the existing template a lot easier to read and fixes the confusion I had myself rereading the template (again).
- We fixed the possible suboption types (which was limited to 'bool' only)

* Use latest stable instead of devel docs

lib/ansible/utils/plugin_docs.py

@@ -69,6 +69,13 @@ def add_fragments(doc, filename, fragment_loader):
                     doc['notes'] = []
                 doc['notes'].extend(notes)
 
+        if 'seealso' in fragment:
+            seealso = fragment.pop('seealso')
+            if seealso:
+                if 'seealso' not in doc:
+                    doc['seealso'] = []
+                doc['seealso'].extend(seealso)
+
         if 'options' not in fragment:
             raise Exception("missing options in fragment (%s), possibly misformatted?: %s" % (fragment_name, filename))