Document template testing; improve related docs

- Document template testing repos
- Introduce dom0 testing repos
- Cross-link dom0 and domU testing repo sections
- Link to template testing section from related sections
- Add pointer to installing templates
- Minor improvements and fixes

Thanks to @GWeck for pointing out the need for this in:
https://forum.qubes-os.org/t/fedora-34-template-available-for-testing/4904/4

user/templates/templates.md

@@ -89,7 +89,7 @@ Certain templates come preinstalled with Qubes OS. However, there may be times
 when you wish to install a fresh template from the Qubes repositories, e.g.:
 
 * When a template version you're using reaches
-  [end-of-life](/doc/supported-versions/).
+  [end-of-life](/doc/how-to-update/#upgrading-to-avoid-eol).
 * When a new version of a template that you wish to use becomes
   [supported](/doc/supported-versions/).
 * When you suspect your template has been compromised.
@@ -118,6 +118,9 @@ If you receive the message that no match is found for
 `qubes-template-<DISTRO_NAME>-<RELEASE_NUMBER>`, see
 [here](/faq/#when-i-try-to-install-a-template-it-says-no-match-is-found).
 
+If you wish to install a template that is in testing, please see
+[here](/doc/testing/#templates).
+
 ## After Installing
 
 After installing a fresh template, we recommend performing the following steps:
@@ -242,7 +245,7 @@ new template:
 
 The following sections cover advanced topics pertaining to templates.
 
-### Inheritance and Persistence
+### Inheritance and persistence
 
 Whenever an app qube is created, the contents of the `/home` directory of its
 parent template are *not* copied to the child app qube's `/home`. The child app