docs: explain using refs in a config file

[tatomyr]

What/Why/How?

Added docs for $refs in the config file: #1345.

Merge this alongside the release.

Reference

Testing

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• Code is linted
• Tested with redoc/reference-docs/workflows (internal)
• All new/updated code is covered with tests

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• Security impact of change has been considered
• Code follows company security practices and guidelines

[changeset-bot]

⚠️ No Changeset found

Latest commit: b184e74

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[github-actions]

Command	Mean [s]	Min [s]	Max [s]	Relative
redocly lint packages/core/src/benchmark/benches/rebilly.yaml	1.065 ± 0.045	1.022	1.156	1.00 ± 0.05
redocly-next lint packages/core/src/benchmark/benches/rebilly.yaml	1.063 ± 0.017	1.041	1.085	1.00

[github-actions]

Coverage report

St.❔	Category	Percentage	Covered / Total
🟡	Statements	76.31%	4100/5373
🟡	Branches	66.29%	2183/3293
🟡	Functions	68.42%	663/969
🟡	Lines	76.51%	3847/5028

Test suite run success

653 tests passing in 94 suites.

Report generated by 🧪jest coverage report action from b184e74

[HCloward]

Made a couple of suggestions, otherwise looks good to me.