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/advanced-topics/how-to-install-software-in-dom0.md

@@ -115,7 +115,11 @@ sudo dnf remove anti-evil-maid
 
 ## Testing repositories
 
-There are three Qubes dom0 [testing](/doc/testing/) repositories:
+If you wish to install updates that are still in [testing](/doc/testing), you
+must enable the appropriate testing repositories.
+
+**Note:** The following repos are in dom0. For template testing repos, see
+[here](/doc/how-to-install-software/#testing-repositories).
 
 - `qubes-dom0-current-testing` -- testing packages that will eventually land in
   the stable (`current`) repository
@@ -136,6 +140,8 @@ sudo qubes-dom0-update --enablerepo=qubes-dom0-unstable
 To enable or disable any of these repos permanently, change the corresponding
 `enabled` value to `1` in `/etc/yum.repos.d/qubes-dom0.repo`.
 
+For testing new templates, please see [here](/doc/testing/#templates).
+
 ## Contributed package repository
 
 Please see [installing contributed

user/downloading-installing-upgrading/testing.md

@@ -65,6 +65,26 @@ older versions has been deemed stable enough. While this limitation can be
 inconvenient, the benefits outweigh the costs, since it greatly simplifies the
 testing and reporting process.
 
+## Templates
+
+How to test [templates](/doc/templates/):
+
+* For official templates, enable the `qubes-templates-itl-testing` repository,
+  then [install](/doc/templates/#installing) the desired template.
+* For community templates, enable the `qubes-templates-community-testing`
+  repository, then [install](/doc/templates/#installing) the desired template.
+
+To temporarily enable any of these repos, use the `--enablerepo=<repo-name>`
+option. Example commands:
+
+```
+sudo qubes-dom0-update --enablerepo=qubes-templates-itl-testing
+sudo qubes-dom0-update --enablerepo=qubes-templates-community-testing
+```
+
+To enable or disable any of these repos permanently, change the corresponding
+`enabled` value to `1` in `/etc/yum.repos.d/qubes-templates.repo`.
+
 ## Providing feedback
 
 Since the whole point of testing software is to discover and fix bugs, your

user/how-to-guides/how-to-install-software.md

@@ -13,7 +13,8 @@ title: How to Install Software
 ---
 
 When you wish to install software in Qubes OS, you should generally install it
-in a [template](/doc/glossary/#template).
+in a [template](/doc/glossary/#template). For installing templates themselves,
+see [how to install a template](/doc/templates/#installing).
 
 Advanced users may also be interested in learning how to install software in
 [standalones](/doc/standalones-and-hvms/) and
@@ -79,6 +80,10 @@ updating software in domUs.
 If you wish to install updates that are still in [testing](/doc/testing), you
 must enable the appropriate testing repositories.
 
+**Note:** The following repos are in templates and standalones. For dom0 testing
+repos, see [here](/doc/how-to-install-software-in-dom0/#testing-repositories).
+For testing new templates, please see [here](/doc/testing/#templates).
+
 #### Fedora
 
 There are three Qubes VM testing repositories (where `*` denotes the Release):

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