Add preview documentation section

[zanieb]

Adds a basic documentation section for preview mode based on the FAQ entry and versioning RFC.

I intend to expand this with more details as we play with the user experience here. We could also consider generating an enumerated list of preview features 🤷‍♀️

[github-actions]

PR Check Results

Ecosystem

✅ ecosystem check detected no changes.

[codspeed-hq]

CodSpeed Performance Report

Merging #7281 will degrade performances by 5.33%

_{Comparing zanie/preview-docs (2198bb7) with main (773ba5f)}

Summary

❌ 5 regressions
✅ 20 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	main	zanie/preview-docs	Change
❌	linter/all-rules[numpy/ctypeslib.py]	33 ms	34.8 ms	-5.33%
❌	linter/all-rules[unicode/pypinyin.py]	15.2 ms	15.7 ms	-3.02%
❌	linter/all-rules[numpy/globals.py]	4 ms	4.1 ms	-2.58%
❌	linter/all-rules[pydantic/types.py]	69.5 ms	73 ms	-4.75%
❌	linter/all-rules[large/dataset.py]	156.8 ms	163.2 ms	-3.95%