boards/nRF: Make documentation more consistent and updated

[crasbe]

Description

The documentation for the Nordic Semiconductor nRF boards is in a somwhat inconsistent shape. This makes it harder to get started with these particular boards. Some features of the boards that RIOT supports are not mentioned in the documentation etc.
And it seems like they suffered a bit of bit rot and some information has to be updated.

• Update OpenOCD flashing information in nRF Common documentation
• Fix broken image in nRF52 documentation (maybe I can make one myself, so it does not rely on external images).
• Add Hardware Overview (such as in nRF52DK) table to all nRF pages.
• Add a section about the onboard flash memory to the nRF52840 page.
• Add a picture to the nRF52840 Dongle page.
• Add information about the limitations of NFC (as described in boards/doc: Explicitly warn when nRF chips are advertised as NFC supporting #20605 ).
• Rename "NRF6310" to "nRF6310" so it can be found next to the other nRF boards... or remove it. Most of the Google entries you can find about it are from RIOT...
• Update the "nRF52 Flashing" section in "nRF52 Common" (see cpu/nrf52: Add Make Option to Enable Pinreset #20775 ).
• Add information about the the debugging with OpenOCD: the debugging process can not be started if the device has been powercycled after flashing.
• Add (temporary) information in the documentation why the nRF52840 DK is not currently supported by OpenOCD (see Issue cpu/nrf52: OpenOCD not working with newer Revision Silicon and nRF52840 J-Link #20968 ).

Please do let me know if you see more potential tasks to do.
I'll get started on this next week(-ish).

Useful links

https://doc.riot-os.org/group__boards__common__nrf52.html
https://doc.riot-os.org/group__boards__nrf52dk.html
https://doc.riot-os.org/group__boards__nrf52840dk.html
https://doc.riot-os.org/group__boards__nrf52840dongle.html
https://doc.riot-os.org/group__boards__nrf52832-mdk.html
https://doc.riot-os.org/group__boards__nrf51dk.html
https://doc.riot-os.org/group__boards__nrf51dongle.html

Stuff about the nRF6310:
https://doc.riot-os.org/group__boards__nrf6310.html
https://developer.nordicsemi.com/nRF51_SDK/nRF51_SDK_v4.x.x/doc/html/group__nrf51__setups.html#nrf51_setup_nrf6310
https://web.archive.org/web/20191031043659/http://www.mommosoft.com/blog/category/nrf51822/

[crasbe]

@mguetschow the last ToDo item was meant for the boards that currently have no information about NFC at all (like the nRF52DK, nRF52840DK, ..?)
So it is not quite done yet, but I added a reference to your PR so I could keep it in mind to add it :D