docs: move alternative ways to install mise to a separate page

[hverlin]

As suggested in #2440, the getting-started page might be too intimidating for someone new to mise.

2/3rd of the page is filled up with: Alternate Installation Methods, Shells, and Uninstalling. I think it makes sense to link to this content in other pages but for the majority of users this is only adding noise and making them want to leave the docs or search for more examples.

To address this, this PR adds an installing-mise.md page which lists all the ways that mise can get installed.

A little into the page it offers you the choice about activate vs shims vs none. Most people new to Mise won't want to make choices like this right away since it requires research to understand the consequences.

There is very little explanation about what activate actually does at this point. It doesn't need to be in-depth like the FAQ, just something like "it updates your env vars and path every time your prompt is run to ensure you use the correct versions"

Moved the shims activation method to shims and added a short explanation.

Other changes:

• Made all the code blocks easier to copy-paste by removing the $
• Updated the using mise section of the getting started so that users try to use both mise exec -- node -v and node -v
• Add section on how to set up the autocompletion

[hverlin]

This tip is nice, however I would perhaps move it to another page?

[hverlin]

I don't use fish. However, I still tried it to make sure it works. I am getting the following error (even with usage installed)

root@2c295de84df0 /# source ~/.config/fish/completions/mise.fish
~/.config/fish/completions/mise.fish (line 6): 'return' outside of function definition
    return 1
    ^
from sourcing file ~/.config/fish/completions/mise.fish

removing the return 1 fixes it

[jdx]

you don't source completion files in fish