Update README to remove building with nw.js

[McGiverGim]

Updates and completes the README to make clear that we are moving to PWA, updates the "master" snapshots URL and the instructions for building.

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[haslinghuis]

[McGiverGim]

I didn't notice it... moved upper and updated.

[sonarcloud]

Quality Gate failed

Failed conditions
9.7% Duplication on New Code (required ≤ 3%)

See analysis details on SonarCloud

[nerdCopter]

off-subject:
i noticed that https://app.betaflight.com/ and https://master.dev.app.betaflight.com/ are at the same commit (after browser refresh)
will there be a separation of stable vs master at some point @blckmn ?

[nerdCopter]

maybe the new PWA information should be top of readme with "note" alert/admonition syntax.

https://github.com/orgs/community/discussions/16925

[McGiverGim]

The idea of this PR is to create a transition readme, for this reason I have not added the future stable URL.
For the same reason I restricted the PWA information to where is needed, to not confuse non developers users. For this reason I think it's good at this point not add this info on the top.