docs: add learning notes for a developer who are completely new fresh to OpenHands

[georgesheth]

share learning note to understand how OpenHands works

• [add learning notes on how OpenHands backend works by studying on its initialisation, interaction and knowledge management]
• [add these notes as a new category into Sidebar to be shown on OpenHands Docs]

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As a new developer who want to contribute to the development of OpenHands, he/she needs to understand how exactly OpenHands works in the background.
I take this initiative to study OpenHands with zero knowledge and make notes during my study, and now decide to share.
It could also take some other new developers to spend much efforts to familiar with OpenHands in order to do some code contribution. So why not share them ?
During my study of OpenHands Doc, it did not provide much information on how Backends works exactly, these information are embedded in the codebase, so I summary my learning notes to others and hopefully they will be useful to other new developers.
It did not touch on any other existing code, but only add new docs pages. The resulted changes are looks like as this attached images.

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not link to any issues yet.

[georgesheth]

@enyst or @mamoodi @openhands-agent you wanna a review ? I saw you have contributed lots to the docs.

[mamoodi]

@enyst or @mamoodi @openhands-agent you wanna a review ? I saw you have contributed lots to the docs.

Thanks for your contribution! I'm not as familiar with the inner workings of OpenHands so I won't do a good job of reviewing.

I am wondering how this will be kept up to date as the project is moving and changing really fast. Have you thought about that as well?

[openhands-agent]

OpenHands started fixing the pr! You can monitor the progress here.

[enyst]

Thank you for this! I do think we have more improvements to make, to get openhands more friendly to new contributors.

I love the diagram in your screenshot! In the same time, Mamoodi makes a good point, and perhaps we don't shouldn't duplicate actual source code in the documentation. Instead, we actually have it automated in this Python API section: https://docs.all-hands.dev/modules/python/openhands/events/stream

How about we just link to some of those classes/files, for the most important types you uncovered?