Describe your changes

Draft documentation for the parallel=True parameter on @st.fragment, intended as a reference/handoff for a contributor to submit as a PR to streamlit/docs.

This PR contains a single markdown file with proposed changes to 5 pages in the docs repo:

1. Concepts page (fragments.md) — new "Run fragments in parallel" section, thread safety guidance, restricted commands, plus updates to use cases, execution flow, limitations, and run_every interaction
2. API reference (fragment.md) — SEO keyword additions (parallel, concurrency)
3. New tutorial — "Speed up your app with parallel fragments" — full step-by-step walkthrough
4. Menu — new tutorial entry under Execution flow / Fragments
5. Release notes — verified already covered (no change needed)

For the contributor

The draft is AI-generated and needs human polish before submitting to streamlit/docs. Key suggested tasks:

• Run the tutorial code end-to-end and verify every step works
• Rewrite prose in your own voice to match the Streamlit docs tone
• Verify version number (1.58.0) and all internal doc slugs
• Add screenshots/GIFs (the existing fragment tutorials have them)
• Confirm callout tag style (<Tip>, <Warning>, <Note>) with the docs team
• Check if a runnable example source should be added under python/api-examples-source/

Testing Plan

• No tests needed — this is a documentation reference file, not a code change.

Contribution License Agreement

By submitting this pull request you agree that all contributions to this project are made under the Apache 2.0 license.