[skip changelog] Document platform bundled libraries

[per1234]

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• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce?

Docs update

• What is the current behavior?

Bundled libraries are a very important component of most Arduino platforms, but this capability only has a brief mention in https://arduino.github.io/arduino-cli/dev/platform-specification/#referencing-another-core-variant-or-tool and https://arduino.github.io/arduino-cli/dev/sketch-build-process/#location-priority, with no explanation of what they are or how they are used.

Even fairly advanced Arduino users are often confused by the fact that Arduino libraries are stored in multiple locations.

• What is the new behavior?

Platform bundled libraries are documented in the Arduino platform specification.

• Does this PR introduce a breaking change?

No.