Update and improve the bootloader documentation

[JF002]

Improve the documentation of the bootloader, add background information, info on design choices, remove outdated content.

Move the doc from /bootloader to doc/bootloader and remove outdated/unmaintained files from /bootloader.

[JF002]

Great cleanup.

Thanks :) And thanks for the review!

Maybe this document should be in the bootloader repo and we simply link to it from here?

You're probably right. There was already some documentation in InfiniTime regarding the bootloader, but it makes more sense to move it to the bootloader project. I suggest we continue the review in this PR, and more it later on?

~I propose we split paragraphs using semantic linefeeds https://rhodesmill.org/brandon/2012/one-sentence-per-line/. This will make diffs much simpler.~ Rather this is what I maybe would've done, but maybe we can discuss this later when we decide on a markdown format.

Mhm this seems like a good idea. I'll give it a shot and see how it looks like ;)