Documentation: Target branch for merge requests (dev or main branch)

Hello. The ([current documentation](https://nalgebra.rs/community)) states

> If you intend to work on the source code of nalgebra, you should start by forking the repository. Once you are done making modifications to your own copy of nalgebra, you have to create a pull request targeting the dev branch so that your contribution can be reviewed, commented, and eventually merged.

I only found the [dev-0.32.x branch](https://github.com/dimforge/nalgebra/tree/dev-0.32.x) and all of the recently closed merge requests seemed to be created against the main branch. Could we update the documentation to reflect the current process?

Start with the community documentation page linked in the issue and compare its target-branch guidance with the dev-0.32.x and main branches and recently closed merge requests. Done means the documentation accurately explains whether new merge requests should target dev or main.

Written by the indexing model from the issue text.