Add developer documentation

[jwuensche]

This PR contains some initial work to provide documentation to developers wanting to work on haura.

It aims to contain both an overview of the conceptual level of the storage design and internal implementation related to these.
Also, new features can be drafted here and reasoning added as to why some implementations have been chosen over another (picture RFC like).

What has been done?

• Navigation Page
• betree
  • betree crate concept
    • Database, Dataset, Objectstore
    • B-epsilon tree
    • Data Management
    • Storage Pool (not fully complete - Segments)
  • betree modules
  • betree known bugs
• bectl
  • purpose
  • usage
• julea-sys
  • generation
• julea-betree
  • build
  • validation

[jwuensche]

I've done some changes to the structure of the documentation. First, it now uses mdbook to document the building and getting started of the stack in a more friendly way. Second, a workflow has been added which (when present on the main branch) automatically builds and deploys the most recent version of the docs as an html book, which is then reachable under https://julea-io.github.io/haura.

[michaelkuhn]

I found a few small typos etc. that we should fix before merging.

[michaelkuhn]

LGTM