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Since some of the things became migrated/outdated, and some is simply not documented yet, so we might want to refresh our internal documentation about them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/
• che-blog

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.

Document process of releasing and updating each of the repositories (ex. when Che release happens)

Describe alternatives you've considered

Additional context

No response

[mkuznyetsov]

CONTRIBUTING.md could use a summary of all existing workflows in the project, describing how they are linked between each other