[docs] Simplify Nushell integration

[texastoland]

Description

Simplifies Nushell integration instructions (docs only) in the following ways:

• Saves the script to a standard cross-platform path
• Just use starship.nu instead of arbitrary path
• Provides runnable commands
• Explains why it's necessary
• Explains how to update

Motivation and Context

Motivated by jdx/mise-docs#46 for consistency between tools.

Checklist:

• I have updated the documentation accordingly.
• I have updated the tests accordingly (N/A).

[davidkna]

It hasn't been more stable lately, but nushell has had many breaking changes and our init script also gets updated now and then, so I would like to avoid having the init script static at this time.

[texastoland]

I would like to avoid having the init script static at this time.

It was already static. Here's the relevant diff:

-   starship init nu | save -f ~/.cache/starship/init.nu
+   starship init nu | save $starship_path --force

I included a link to explain why it's required:

+   Since Nu does [not support `eval`](https://www.nushell.sh/book/how_nushell_code_gets_run.html#eval-function) you must save the initialization script:

[davidkna]

FWIW, it is not necessarily static between different versions of starship (or with different starship binary locations).

More importantly, if the file doesn't update itself, it will make handling user issues more difficult and will cause additional strife on breaking nushell updates. Given all that, I'd rather not accept this PR.

[texastoland]

@davidkna There's a misunderstanding. I didn't change that recommendation. It is and always was "static" because Nu has never supported any eval-like functionality (doc link above). That confusion is why my PR exists.

• Saves the script to a standard cross-platform path
• Just use starship.nu instead of arbitrary path
• Provides runnable commands
• Explains why it's necessary
• Explains how to update

These changes explain the conceptual reason and simplify installation for Nu users. I started a thread on your Discord server for clarification.