Add `bootloader` documentation.

[playfulFence]

We need to document things regarding the bootloader. Like how to build a custom one in esp-idf, how to use it with esp-hal and stuff like that. However, we need to: A) Find a suitable place where and how to put this documentation so that it is clearly visible. B) Keep in mind that relatively soon there might be some updates in terms of bootloader in our driver.

[bugadani]

IMO this should be part of the book and/or future guides, not HAL documentation. We should mention what bootloader we use/expect, but it's somewhat tangential here.