docs(config): fix CODE_EXECUTOR_SECURITY_THRESHOLD semantics and default

[Artur749]

Summary

Corrects documentation for CODE_EXECUTOR_SECURITY_THRESHOLD to reflect the semantic fix from
EPMCDME-13877: values now behave as named (LOW = permissive, HIGH = most restrictive).
Adds a version-free migration note for operators who explicitly pinned LOW for strict enforcement.

Changes

• Updated default in env-var table: "LOW" → "HIGH"
• Rewrote Security Threshold callout with correct SAFE < LOW < MEDIUM < HIGH strictness order
• Added migration note inside the Security Considerations callout
• Added Upcoming Release section to release notes with breaking change entry (version placeholder — replace heading/date once release is announced)
• Added 2 FAQ entries covering threshold semantics and migration steps

Testing

• Tested locally with npm start
• All pages render correctly
• Internal links work
• Sidebar navigation works

Quality Checks

• npm run check passes (typecheck + lint + commitlint)
• npm run build passes (no broken links)
• No MDX compilation errors
• Commit messages follow Conventional Commits

Additional Notes

Breaking behavioral change: any deployment explicitly setting CODE_EXECUTOR_SECURITY_THRESHOLD=LOW
for strict enforcement must change it to HIGH. Deployments relying on the default are unaffected.

The release notes section uses a placeholder heading (### Upcoming Release {#upcoming}) with a
TODO comment — replace with the real version heading once the release is announced.

Related MR: https://gitbud.epam.com/epm-cdme/codemie/-/merge_requests/4340