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

Set default pins for peripherals per datasheet (earlephilhower#103)

Using the official Raspberry Pi Pico datasheet and the Adafruit Feather
RP2040 schematic, set the default pins for peripherals to match.

Fixes earlephilhower#92

Remove api from include path, add redirects

Can't have cores/rp2040/api in the include path because the Arduino API
dir has a "String.h" file.  On Windows, because it is case-insensitive
normally, this overrides the POSIX "string.h" header leading to bad
stuff.

Add manual redirect includes for commonly accessed headers in the
cores/rp2040 path instead.

Fix JSON, Arduino 2.0 beta

Missing Size field was causing the Arduino 2.0 beta to reject the JSON.