boards/nrf52840dongle docs: Introduce "quick start" section

[chrysn]

… to mention the board name; also to point to a good first example to use with this board.

Contributes-To: #15654 (which rightfully stated that the board name was not mentioned in the docs, and users should not need to pull it out of the URI.)

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Apart from fixing #15654 for the exemplary case, this might also serve as something to generalize. So far, we barely have per-board quickstarts; only the msbiot has something similar (but embedded in the middle of the page and more referring to the family specific wiki page). The conclusions gathered here might serve as input to future structured data on boards.

[chrysn]

Follow-up commit added as the static checks were not happy with the (unmodified) heading level of the nrf52840 docs; I assume the CI checks doxygen warnings were set to stricter since this was originally written.

[chrysn]

Removed the follow-up commit again: That wasn't it. Rather, the doxygen version in CI seems to misunderstand the named heading. With version 1.8.20 locally at my site it works (unlike the build artifact), and does not show the error which static tests complain about.

[benpicco]

@chrysn ping 😉

[chrysn]

Wasn't up there in the list yet ;-) -- but thanks for the nudge. Rebased and now including @jeandudey's solution, let's see what CI makes of it. (My local doxygen is known to produce different results, sadly).

[miri64]

(My local doxygen is known to produce different results, sadly).

Seems to work now.

[miri64]

Just noticed: there is a typo in the commit message. Would you mind fixing it?

[chrysn]

Thanks, fixed. (And apologies to @maribu whose #16023 I've CI-toggled to get this 20-second job to run before the 20-minute one, just so I can merge it right away).

[miri64]

Just noticed: there is a typo in the commit message. Would you mind fixing it?

Puts "commit message spell checker" on TODO list