Document che-docs/che-website build, release and publishing workflows

[mkuznyetsov]

Is your task related to a problem? Please describe

There were some problems with Che Docs/Che Website recently, and it wasn't clear at times how to go about them.
Since some of the things became migrated/outdated, and some is simply not documented yet, so we might want to refresh our internal documentation about them

Describe the solution you'd like

the essential information about

• che-docs, its pipelines for building, PR checking and release
• che-website, its publishing and updating on a live website https://eclipse.dev/che/

this should give us a good idea, how the docs/site updating process happens from start to finish, and how to troubleshoot problems on our, or Eclipse's end.

Describe alternatives you've considered

Additional context

No response