Add Github CI platformio

[ewowi]

Basic setup see MoonModules@080a957

Earlier PR submitted (but not merged yet), try to combine with this: theelims#65

See also #25

[ewowi]

Upstream (ESP32-SvelteKit) recently added new code to generate merged bins (see build/merged folder). I excluded them from the github in .gitignore as that caused all the bins (or the the bins you recompiled) in each commit to be updated which I think you should not want. Ideally the bins should be generated within github actions when you do a commit. In WLED I saw they are even moved to a nightly build pre-release - that would be even more beautiful.

[ewowi]

Currently webinstaller bins are in /docs/general/installer:

This 'works' but is a temporary solution

cc @Flavourdynamics

[Flavourdynamics]

Okay, so in the future these builds should be automatically integrated. For the short term, I will document the procedure:

1. Create your new board and def (partition, JSON, ini)
2. Build and place compiled bin into the installer, update manifest

Let me know if I missed any key steps as I write this up.