Update SPFx docs for gulp-based toolchain deprecation ▶ heft-based toolchain in SPFx v1.22 GA release

[andrewconnell]

Important

This is a tracker for content updates related to the SPFx v1.22 GA release that includes the deprecation of the legacy gulp-based toolchain in favor of the new Heft-based toolchain. The toolchain shift impacts lots of docs across this repo.

🚧🚧 This is an ongoing project... use this to monitor progress 🚧🚧

update heft toolchain docs for GA

• update & fix heft content b7f8b96
• remove "preview" red warning on heft docs 18242ba
• update timestamp to GA date 18242ba
• update migration doc for GA 28e8032

gulp toolchain docs

• rename to legacy 660476b 806a6c9
• dupe existing "getting started > set up dev env" & edit 806a6c9

spfx overview

• fix inaccuracies 2bef10c

getting started > setup dev env

• convert to heft fe2808a
• purge unnecessary stuff (on-prem) fe2808a

on-prem docs

• point to gulp-toolchain 9b35207