[DOCS] Document how to install optional dependencies for Salt

[dehnert]

Description
I was previously running 3004.1 and had working states using the virt module. That requires the libvirt Python module, so I installed the python3-libvirt package, which worked. I then installed 3006.1 using the instructions, and the states complained that the virt module is missing. Given that I had the Python libvirt module installed, this was confusing. Judging by https://docs.saltproject.io/salt/install-guide/en/latest/topics/upgrade-to-onedir.html#upgrade-to-onedir, I think the recommended approach is to install with pip and that will install in Salt's Python install, not the global one, but I think it would be useful to be more explicit about that. I'm used to using Ubuntu packages to install Python libraries when available, and no longer supporting that feels like a big change.

Suggested Fix
Add improved documentation of how to install optional dependencies

Some places I could see this being helpful:

• A topic page somewhere with a recommendation (the upgrading to onedir page actually has a decent start here, but I expect this will impact people who aren't upgrading -- or who are migrating their states but not upgrading any individual machines, who might not think to read that page)
• On the main install overview page -- perhaps a new step between steps 2&3 that says "install optional dependencies", and links to a topic page
• On the various per-OS install guides, like https://docs.saltproject.io/salt/install-guide/en/latest/topics/install-by-operating-system/ubuntu.html
• On various module pages with optional dependencies, link to docs on installing them

Type of documentation
This could be Salt documentation, Salt modules, the Salt Repo or the Getting Started guide.

Could impact all of these

Location or format of documentation
https://docs.saltproject.io/salt/install-guide/en/latest/topics/install-by-operating-system/ubuntu.html is what I was following

Additional context
Add any other context or screenshots about the feature request here.

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[barbaricyawps]

@dehnert , I think you're not the only one who is confused by this new approach. I'll work with the team to see if we can make the instructions more clear about installing into the Salt Python rather than the global one.

[OrangeDog]

The main issues continue to be:

• The only mention of onedir dependencies is in the upgrade instructions, which new users will not look at.
• All the module documentation assumes a regular (classic) system install if gives instructions for its dependencies. IMO they should not give instructions, only detail what the dependencies are.

[ITJamie]

again that notice about needing to use salt-pip or needing to update states to have bin_env declared with installing salt pip packages needs to be a warning block at top of all the install guides

[barbaricyawps]

I have a Salt install guide merge request open to resolve this issue if any of you want to check it out and provide comments: https://gitlab.com/saltstack/open/docs/salt-install-guide/-/merge_requests/119

I recommend looking at the rendered doc previews I link in the merge request to see how they will look in the final docs.

Be aware that adding links to the install docs from all of the module docs is out of scope for now, but I've noted that as a tech debt item we might want to address as part of our module documentation improvement epic.

[barbaricyawps]

I merged in that merge request. I'm going to close this issue as fixed. Let me know if you need anything else!