[FEATURE] Better AVR documentation

[linguini1]

Is your feature request related to a problem? Please describe.

The AVR documentation lacks information about flashing many of the boards, especially the ATMega boards. The ATMega chip documentation is also not particularly well-organized.

Part of #16278 number 3

Describe the solution you'd like

• General information about installing the AVR toolchain for all systems in the main AVR documentation page, with back-links from each board support page so users can easily find the information they need for building NuttX.

• ATMega documentation separated into sub-directories for different ATMega chips, instead of one main ATMega page.

• All AVR boards supported in the boards/avr/ directory to have a documentation page, following the updated template for board support docs

• All board support pages to link to some information about how to flash the board (i.e. avrdude incantations at least, maybe special info for unique boards or ICSP programming). If the information is generally applicable (i.e. how to use avrdude but just swapping out the -p option) it could be included at the higher level AVR doc page and linked to.

Describe alternatives you've considered

No response

Verification

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