PWA Studio 12.1.0 release notes (#3537)

* WIP release notes

* Changes from draft feedback

* Updated links

* Updated links, for real this time

* Finished release notes draft

* Finished release notes draft

* Finished release notes draft

* Updated breaking changes and other munged topics

* Spelling fix

* Changed 'extension' to 'app' to cover the changes we add to the metapackages

* Made few table formatting changes

* Updated to emphasize new metapackages and changed breaking changes to 'Metapackage updates'

* Another update to emphasize the need to install new metapackages

* Another update to emphasize the need to install new metapackages - refinements

* Added metapackage installations to the Updating from preview versions section