Clarify installation docs

[wren]

Documentation Change

Affected Page(s)

• https://jrnl.sh/installation/#installation

What Could Be Better?

The documentation isn't super clear about what to do with pipx. There are sometimes multiple ways to install pipx, and not all of them might work. We had an issue come up recently where a user installed pipx through apt, and had lots of troubles (#1087). We should clarify this a bit in our docs instead of just linking to other docs. At very least, there should be a note about following the link to see installation instructions there.

Other Information

This may or may not also affect the homebrew installation instructions.