Update Documentation for schema 1.2

[Trenly]

This cleans up some of the files and documentation in the Repo.

• Removes old and unused PR Template
• Removes old and disabled spelling action which was disabled due to a CVE and has since been replaced
• Removes fabricbot jobs which are disabled
• Updates references to schemas to be version 1.2.0
• Changes the reference for schema documentation to the more-complete docs over at winget-pkgs, and removes the incomplete and outdated doc here
• Updates references to current milestones
• Removes references to a specific version of Visual Studio when linking directly to the download page

cc @denelon for label Area-Docs please

Microsoft Reviewers: Open in CodeFlow

[denelon]

Reference:

• https://github.com/microsoft/winget-pkgs/pull/69104

@ryfu-msft this should also be updated when we start accepting 1.2 schema manifests.

[JohnMcPMS]

@denelon should be the one to sign off on this.