[DOCS] Document how to install optional dependencies for Salt

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.

Start with the upgrade-to-onedir page and the Ubuntu install guide linked in the issue, then review the install overview page for where optional dependencies should be introduced. Update the relevant documentation so users can identify and install dependencies in Salt’s Python environment, with guidance covering the general install flow and per-OS instructions.