Installation guide improvements

[amiv1]

Hello, Beets community and maintainers of this awesome project.

I have some ideas how to improve and simplify the current installation guide that we have and I'd like to hear your ideas and opinions about it.

The current guide already mentions that pipx is the recommended way to install Beets, which is a good start already and very helpful information. Here's how, in my opinion, we can make it even better and reduce friction for beginners:

1. Remove "Using a Package Manager" section

The page already has a huge warning informing about multiple disadvantages of this approach. Important part of Beets is its plugin ecosystem, and usually package manager installations provide terrible experience when it comes to plugin installation.

With such a good alternative as pipx, there may not be much benefit in even mentioning this approach. I suggest removing it completely. Instead, we could describe how to install pipx for each distro if needed, though I think referencing https://pipx.pypa.io/stable/installation/ page is good enough.

Though if you believe we should keep it, we can move this information to the very bottom of Q&A section.

2. Add information how to install plugins with pipx installation

To make it clear how to install plugins and maintain the pipx installation, we could add a new section in Q&A which will demonstate it.

Example:

Install beets with extras (for built-in plugins):

pipx install "beets[lyrics,lastgenre]"

If you already installed beets, reinstall with the new spec:

pipx install --force "beets[lyrics,lastgenre]"

Inject additional packages into the existing beets environment (useful for third-party plugins):

pipx inject beets <package-name>

To upgrade Beets and plugins:

pipx upgrade beets

[amiv1]

Done in #6447