feat(doc): document empty vendor and arch in FQBN

[dankeboy36]

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• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)
• configuration.schema.json updated if new parameters are added.

What kind of change does this PR introduce?

The spec should be aligned with the implementation; empty vendor and architecture parts are allowed in the FQBN.

This PR updates the documentation to mention the empty vendor and architecture parts in the FQBN.

What is the current behavior?

The docs do not mention the empty vendor/architecture cases but the CLI supports them:

arduino-cli/internal/arduino/cores/fqbn_test.go

Lines 33 to 56 in 870a48f

	// Allow empty plaforms or packages
	b1, err := ParseFQBN("arduino::uno")
	require.Equal(t, "arduino::uno", b1.String())
	require.NoError(t, err)
	require.Equal(t, b1.Package, "arduino")
	require.Equal(t, b1.PlatformArch, "")
	require.Equal(t, b1.BoardID, "uno")
	require.Zero(t, b1.Configs.Size())
	b2, err := ParseFQBN(":avr:uno")
	require.Equal(t, ":avr:uno", b2.String())
	require.NoError(t, err)
	require.Equal(t, b2.Package, "")
	require.Equal(t, b2.PlatformArch, "avr")
	require.Equal(t, b2.BoardID, "uno")
	require.Zero(t, b2.Configs.Size())
	b3, err := ParseFQBN("::uno")
	require.Equal(t, "::uno", b3.String())
	require.NoError(t, err)
	require.Equal(t, b3.Package, "")
	require.Equal(t, b3.PlatformArch, "")
	require.Equal(t, b3.BoardID, "uno")
	require.Zero(t, b3.Configs.Size())

What is the new behavior?

The new doc includes the special empty parts case: https://github.com/dankeboy36/arduino-cli/blob/feat-doc-fqbn/docs/FAQ.md#whats-the-fqbn-string

Does this PR introduce a breaking change, and is titled accordingly?

No.

Other information

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.38%. Comparing base (870a48f) to head (0a164a8).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2550   +/-   ##
=======================================
  Coverage   69.38%   69.38%           
=======================================
  Files         205      205           
  Lines       20162    20162           
=======================================
  Hits        13989    13989           
  Misses       5050     5050           
  Partials     1123     1123           

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