Update website and docs URLs

Steps to reproduce:

1. This GitHub repository has https://github.com/dotnet/command-line-api/wiki (its own Wiki) specified as website.
2. If you click that link you'll see a page that tells you: _The contents of the wiki have moved to the [docs](https://github.com/dotnet/command-line-api/tree/main/docs) folder in the main repo._
3. If you follow the link you'll see a README that tells you: _The System.CommandLine documentation can now be found at [Microsoft Docs](https://docs.microsoft.com/en-us/dotnet/standard/commandline/)._
4. If you click again Microsoft will redirect you to https://learn.microsoft.com/en-us/dotnet/standard/commandline/

Possible improvements:

- [ ] Change GitHub repository website to https://learn.microsoft.com/en-us/dotnet/standard/commandline/
- [ ] Change Wiki landing page link to the same or disable Wiki as it is not used anymore.
- [ ] Update link in `docs/readme.md` to https://learn.microsoft.com/en-us/dotnet/standard/commandline/

Start with the GitHub repository website and Wiki settings, then inspect docs/readme.md, which are the locations named in the issue. Update the listed links to the Microsoft Learn URL or disable the unused Wiki, and verify that each repository documentation entry points directly to the current page.

Vom Indexierungsmodell aus dem Issue-Text verfasst.