doc: update snaps and releases documentation

[minaelee]

This PR:

• Adds a new Releases and snaps reference page, similar to the recently added LXD documentation Releases and snap page. Updates/adds links to this page from other pages.
• Enables use of a sustitutions.yaml file similar to LXD's for facilitating documentation updates of the current LTS and feature tracks for MicroCloud.
• Moves the information about holding and synchronizing updates from the Manage the snap page to the Updates and upgrades page.
• Improves and adds to the Manage the snap and Updates and upgrades how-to guides.
• Updates the How to get support guide to align with LXD's how to get support guide.
• Updates the How to install MicroCloud guide to use the recommended defaults for installation, indicate that specifying a channel is optional, clarify how to handle a previously installed snap, etc.

[roosterfish]

Thanks for this, left a few comments to highlight the differences between LXD and MicroCloud releases and some more smaller details that might be helpful too.

[roosterfish]

I am trying to follow what this change is all about as it doesn't match the commit message.

Can you please move this change (and probably the other ADDPREREQS) into a separate commit?

[roosterfish]

Ah now I see, you require the pyyaml to load the substitutions file. Can you maybe add a comment somewhere?

For what is gitpython used?

[minaelee]

Moved into a separate commit without gitpython but can't confirm that it will work without until after #839 is merged (per tests currently failing due to MicroCeph changes).

[roosterfish]

PR can be rebased now, the other PR just went in.

[minaelee]

Test won't pass until #839 is merged. Will rebase after.

[roosterfish]

LGTM!

[minaelee]

Updated the compatibility matrix to include feature releases:

Supported and compatible releases

:align: center

| Ubuntu LTS | MicroCloud  | LXD        | MicroCeph       | MicroOVN |
| ---------- | ----------- | ---------- | --------------- | -------- |
| 22.04      | 2.1*        | 5.21*      | Squid           | 24.03    |
| 24.04      | 2.1*        | 5.21*      | Squid           | 24.03    |

* For MicroCloud and LXD, the most recent {ref}feature releases <ref-releases-microcloud-feature> (after the last LTS) are also supported.