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/