Re-add pico-sdk/extra submodules to release ZIP (earlephilhower#181)

Update release-to-publish.yml

Try to mount Pico drive if not already present (earlephilhower#175)

Fixes earlephilhower#27

As @vslinuxdotnet discovered, sometimes the Pico firmware update emulated USB
stick does not get automounted under Linux.  Add logic to attempt to use
standard tools to mount it to the uploader, only tried if no drives are
detected normally.

Fix over/underclock support (earlephilhower#147)

Removed needed header from main.cpp during prior cleanup.  Re-add.

Fixes earlephilhower#146

Rationalize Adafruit Feather infra, update docs (earlephilhower#145)

Use same board.txt format as other Adafruit boards.
Add in quick note about the new USB stack to the docs.

Massively reduce generated code and RAM size (earlephilhower#137)

Older builds included C++ locale information in RAM/flash as well as a
bunch of exception code which can't get called since they're not enabled.

Remove them by adjusting final link command and not instantiating them
in main.cpp in the first place.

Blink went from:
> Sketch uses 215604 bytes (10%) of program storage space. Maximum is 2093056 bytes.
> Global variables use 18152 bytes (6%) of dynamic memory, leaving 243992 bytes for local variables. Maximum is 262144 bytes.

To:
> Sketch uses 56112 bytes (5%) of program storage space. Maximum is 1044480 bytes.
> Global variables use 12152 bytes (4%) of dynamic memory, leaving 249992 bytes for local variables. Maximum is 262144 bytes.

For a savings of **155KB of flash** and **6KB of RAM**

Merge branch 'master' of https://github.com/earlephilhower/arduino-pico

Point people to READTHEDOCS for help, not GitHub

Most users w/the Library Manager will get a little nervous with the GH site.
The ReadTheDocs site includes links to the GH site, too, so they can still
find the origin.

Fix memory leak in tone/noTone calls (earlephilhower#125)

Was never deleting the tone object on noTone, so eventually the leaked
objects would fill up heap and cause a crash.

Fixes earlephilhower#121

Look for UF2 drives in additional spots on Linux (earlephilhower#37)

Fixes earlephilhower#28