docs: try to make it clearer that preview is for next-gen products

[lornajane]

What/Why/How?

We received some feedback that it isn't clear enough that the preview command is only for early access users. This pull request upgrades the admonition level in an attempt to make it more likely it will be seen/read.

Check yourself

• [x] Code changed? - Tested with redoc/reference-docs/workflows (internal)
• [x] All new/updated code is covered with tests
• [x] New package installed? - Tested in different environments (browser/node)

Security

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

[changeset-bot[bot]]

⚠️ No Changeset found

Latest commit: 650a8b0e3ca4f38434f1d03f7334806c983104c6

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

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[github-actions[bot]]

Command	Mean [s]	Min [s]	Max [s]	Relative
redocly lint packages/core/src/benchmark/benches/rebilly.yaml	1.007 ± 0.053	0.947	1.117	1.02 ± 0.06
redocly-next lint packages/core/src/benchmark/benches/rebilly.yaml	0.991 ± 0.021	0.952	1.013	1.00

[github-actions[bot]]

Coverage report

St.:grey_question:	Category	Percentage	Covered / Total
🟡	Statements	77.26%	4506/5832
🟡	Branches	67.78%	2495/3681
🟡	Functions	70.98%	751/1058
🟡	Lines	77.47%	4242/5476

Test suite run success

750 tests passing in 105 suites.

Report generated by 🧪jest coverage report action from 650a8b0e3ca4f38434f1d03f7334806c983104c6