Merge dotnetup getting started documentation to main

### Prerequisites

- [x] I have read the [dotnetup documentation](https://github.com/dotnet/sdk/tree/release/dnup/documentation/general/dotnetup).
- [x] I have searched for [existing dotnetup issues](https://github.com/dotnet/sdk/issues?q=is%3Aissue%20state%3Aopen%20label%3AArea-dotnetup).
- [x] I have searched for [existing dotnetup discussions](https://github.com/dotnet/sdk/discussions/categories/dotnetup).

### Issue type

Bug report

### Description

It looks like the getting started doc for dotnetup is only present in a feature branch right now.

https://github.com/dotnet/sdk/tree/release/dnup/documentation/general/dotnetup

I think this is preventing proper SEO. The above doc doesn't appear in the results for me in these searches:
- https://duckduckgo.com/?q=dotnetup+install
- https://www.google.com/search?hl=en&q=dotnetup+install

I think if this doc were merged to the main branch of this repo, then it would probably get indexed better by search engines and it would be easier to find.

This is useful if you are on a machine which doesn't have it installed, you don't remember how to install it, you're just trying to quickly figure out how.

### dotnetup version

_No response_