Split the concepts section into several sub-pages

[corneliusweig]

This PR splits the concepts section into several sub-pages. Each section with a heading of the original version was extracted into a separate page. This works nicely for most sub-pages, but may be too much for the "Workflow" page, which is really small now. Maybe merge that one with "Architecture"? WDYT?

Also, we could revisit the item order here. I changed it from

• Skaffold pipeline
• Skaffold config
• Workflow
• Image repository handling
• Architecture
• Operating modes
• Local development

to

• Workflow
• Architecture
• Operating modes
• Skaffold pipeline
• Skaffold config
• Local development
• Image repository handling

Close #2779

[codecov]

Codecov Report

Merging #2810 into master will not change coverage.
The diff coverage is n/a.

[container-tools-bot]

Error creating deployment, please see controller logs for details.

[container-tools-bot]

Please visit http://34.94.233.95:1313 to view changes to the docs.

[container-tools-bot]

Error creating deployment, please see controller logs for details.

[balopat]

I really like what you did - the reordering as well as the splitting - I don't think that the workflow page is too small - and we can always make it bigger :) Thank you!!!