Update PlatformIO builder script and docs

[maxgerhardt]

• corrects platformio.ini examples in docs (removes platform_packages directives since sourced from registry now)
• adds explicit deprecation warning so that people don't do above and know how to update
• added docs for debugging
• added docs for filesystem (LittleFS) building and uploading
• expanded builder script to respect PIO_FRAMEWORK_ARDUINO_ENABLE_EXCEPTIONS and PIO_FRAMEWORK_ARDUINO_ENABLE_RTTI for enabling exceptions and RTTI, with appropriate docs
• add docs for stack protector
• reworks the include path patch done in Fix IDE include paths / Intellisense #615
  • when compiling, restores compile-commands equivalence with the Arduino IDE (uses -iprefix <prefix> @<file>)
  • when exporting IDE data (for VSCode and other IDEs), places include paths in env[CPPPATH] so that they know about all paths
  • Note: the regression in the Intellisense was actually a Microsoft bug in (https://github.com/microsoft/vscode-cpptools/issues/9434) in which the -iprefix etc. command was not correctly parsed. While this is fixed for VSCode, I still argue that other IDEs which might not parse this flag at all are better off knowing the exact include paths, which this PR enables
• rename of builder-script internal function fetch_fs_size to __fetch_fs_size after PR feedback (needs equivalent change in platform-raspberrypi, which will be done in Patchup maxgerhardt/platform-raspberrypi#8)
• slight modifications to board JSON generation done in feedback to PR review (Arduino-Pico (Earlephilhower) support, PicoProbe Debugging platformio/platform-raspberrypi#36)
  • move variant one layer outwards (more standard)
  • remove duplicated usb_product and usb_manufacturer since they're already in name and vendor, which the builer script will use. (This alternative mechanism is hence only needed in the pico.json and nanorp2040connect.json, which is already done by hand)
  • added newline at end of file

Closes #612.

[earlephilhower]

Thx, LGTM!