Move doc/users/installing/ to doc/install/

[shriyakalakata]

PR summary

This PR addresses the issue #27396 on moving non user guide related content out of /users. This is the second step, moving /doc/users/installing -> /doc/install.

PR checklist

• "closes [Doc]: Move non user guide related content out of /users #27396"
• [N/A] new and changed code is tested
• [N/A] Plotting related features are demonstrated in an example
• [N/A] New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines

[story645]

Probably gonna need a git grep here, but the following will likely need to be updated:
*

• https://github.com/matplotlib/matplotlib/tree/main/doc#readme

• matplotlib/doc/devel/contribute.rst

  Lines 267 to 274 in 3323ae8

  	.. grid-item:: information about the library
  	* :file:`doc/users/installing/`
  	* :file:`doc/project/`
  	* :file:`doc/users/resources/`
  	* :file:`doc/users/faq.rst`
  	* :file:`doc/devel/`

• should probably retain link since that's to stable (and redirect will pick it up) but path:
  

  matplotlib/README.md

  Lines 31 to 35 in e16967f

  	## Install
  	See the [install
  	documentation](https://matplotlib.org/stable/users/installing/index.html),
  	which is generated from `/doc/users/installing/index.rst`

[shriyakalakata]

Hey, I'm not sure why two of the tests are failing. I would appreciate some help

[story645]

@rcomer caught that you probably need to rebase b/c there's been changes to the tests/CI code. The needs-rebase flag didn't get triggered b/c that code doesn't conflict with this PR.

[shriyakalakata]

you probably need to rebase b/c there's been changes to the tests/CI code

@story645 done!

[story645]

is it ok to change this now or should this wait for the other redirects?

[QuLogic]

I think it's fine; it'll only affect the rc page, and once final is out, we'll update the main page to go along with it.

[QuLogic]

Oops, I thought of squashing earlier, but didn't; oh well.