idea for refactor of workflows

[jcbhmr]

no, i dont expect this to be merged as-is. this pr is intended to be EDITED and IMPROVED to align with whatever the official template is willing to accept

this pr would:

• make it so that there's a commented out on: push: main block that can be enabled by the user if they prefer
• explicitly login to docker and use the cli
• use .yml instead of .yaml (.yml is 2x more popular on github search) (somewhat opinionated changed)
• longer names for workflow files (opinionated changed)
• update feature readmes in a separate workflow file (separation of concerns)
• CONTINUOUSLY update feature readmes via automatic commits to prs and main on changes (opinionated change)
• use a dynamic relevant-features matrix to test only changed features on prs and pushes
• still allow workflow_dispatch manual testing of ALL features

expl on why i used docker+devcontainer instead of the official action: its more clear what is happening. not really anything else, the devcontainer/action is a black box that can do like 4 different things if you prod it the wrong way 🤣 also its clear here where and when we are authing with docker instead of magically inside the devcontainers/action action. kinda addresses confusion of devcontainers/action#143

I EXPECT SOME OF THESE THINGS TO BE REQUESTED TO BE CHANGED that's ok 😊 this is just some ideas that i hope some of which can be merged after discussion.

sidenote: idk if you want me to split this into multiple isolated prs or what 🤷‍♀️ idk on ettiquite here for larger-ish changes like this. should i open an issue to discuss this instead? 😵

ref devcontainers/action#197

ideas from https://github.com/devcontainers-community/npm-features and https://github.com/devcontainers-community/features-starter