docs(user-guide): add workflow categories field and filter

[bporodko]

Summary

Documents EPMCDME-12906 (Prod 2.57.0): the optional Categories field in workflow configuration and the Categories filter on My Workflows / All Workflows.

Changes

• create-workflow.md: Categories field (optional, max 3, pre-filled on edit) with screenshot
• workflows-overview.md: new "Filtering Workflows by Category" section with screenshot
• create-workflow-from-template.md, marketplace-publishing.md: field row and clarifying note
• assistant-categories-management.md: category deletion is also blocked when workflows use it
• Two new FAQ entries

Testing

• Tested locally with npm start (verified via npm run build)
• All pages render correctly
• Images display properly
• Internal links work
• Sidebar navigation works

Quality Checks

• npm run check passes (typecheck + lint + commitlint)
• No MDX compilation errors
• No raw angle brackets (<text> must be `<text>`)
• Sidebar references document IDs (not filenames)
• Images stored locally next to content (not in static/img/)
• Commit messages follow Conventional Commits
• No secrets or credentials in documentation

Additional Notes

Source MRs: codemie-ui !1794, codemie !4123. No sidebar changes.