Expand docs with architecture overview

[EiffL]

Summary

• document the internal code architecture
• list new architecture page in the docs index

Testing

• ruff check .
• pytest -q (fails: ProxyError when attempting to download models)

[EiffL]

I have a small comment on the config.py

Also the reason why I didn't include the image and spectum in the manager test is that it was taking too much memory at some point and was crashing the test. At least I think that was the problem... If it works now that's good, but if the tests crash at some point that would probably be why.

[EiffL]

Sorry @LTMeyer but it was on purpose that I didn't include the LegacySurveyImage here. The logic of the codec manager knows how to handle that.

We are only declaring a single Image codec in this config, because there is a single model to load for images. If we have LegacySurveyImage (and HSCImage) the codec manager would load the codec 2 times.

[LTMeyer]

I added it here to fix an issue that occurred when we were running manager.encode(input, "tok_image"), and tok_image was only accessible through LegacySurveyImage. Then I saw it was not the case anymore that we could call the encode with a str instead of a modality.
Should I thus revert it back?

[LTMeyer]

Reverted, but see comment to confirm it was the initial intent.