document build processes

[indirectlylit]

Observed behavior

• It's difficult to understand how to download artifacts - see e.g. https://github.com/learningequality/kolibri/issues/7043
• The high level architecture of our build system is not documented, e.g. Docker, buildkite, etc
• Creating custom builds or build pipelines is not documented, e.g. setting options and installing plugins

Expected behavior

In the developer docs, it should be straightforward for external or internal contributors to learn:

• How does the build system work overall?
• How do I access builds for a PR?
• How do I trigger builds that are blocked?
• How do I access builds for tagged releases?
• How to find builds for tags?

details can be added to at least these pages, but likely we'll want at least one new page fort the general architecture:

• https://kolibri-dev.readthedocs.io/en/develop/development_workflow.html
• https://kolibri-dev.readthedocs.io/en/develop/release_process.html

User-facing consequences

lack of information about build system