fix(NcFormBox): add NcButton to supported components

[ShGKme]

☑️ Resolves

• Design draft example: Mail settings reorganization nextcloud/mail#11910
• Ref: Settings redesign to use new vue components nextcloud/mail#12031
• Additionally: improve documentation

🖼️ Screenshots

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable
• 2️⃣ Backport to stable8 for maintained Vue 2 version or not applicable

[ShGKme]

/backport to stable8

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.17%. Comparing base (8e6c2a6) to head (55e0b84).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #7886      +/-   ##
==========================================
+ Coverage   52.11%   52.17%   +0.06%     
==========================================
  Files         100      101       +1     
  Lines        3170     3174       +4     
  Branches      872      873       +1     
==========================================
+ Hits         1652     1656       +4     
  Misses       1271     1271              
  Partials      247      247              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[ShGKme]

/backport to stable8