docs(integrations): clarify entry accepts any fragment key + FAST-v3 wrapper requirement

[janechu]

What

Two small clarifications carried forward from the discussion on #299 (which was closed in favour of inline updates rather than a separate recipe page, per maintainer preference):

1. Both docs/guide/integrations/node.md and docs/guide/integrations/rust.md now explicitly state that entry / entry_id accepts any fragment key in the built protocol, not just "index.html". Adds a webui inspect pointer for listing the valid keys.

2. Calls out the FAST-v3 wrapper-file requirement: an <f-template name="my-component"> block lives in the protocol's component map (not the fragment map), so it cannot be passed as entry directly. Adopters trying to render a single FAST-v3 component into an embedded host were hitting MissingFragment and had to read source to figure out why.

Also adds a one-line distinction between render_partial (returns JSON for the WebUI client-side router during in-app navigation) and render / WebUIHandler::handle with entry (returns raw HTML for any host), since these two APIs were being conflated in the discussion on #299 and the existing docs don't make the distinction explicit.

Why

The existing integration pages say entry is "Fragment ID to start rendering from" but only ever show entry: 'index.html' in examples. An adopter has to read the source to figure out (a) that they can pass any fragment, (b) how to list valid keys, and (c) why their FAST-v3 <f-template> block doesn't work as an entry. These three points together are ~20 lines of doc that pay back the source-reading exercise.

How I tested

• cargo xtask license-headers ✔
• cargo fmt --all --check ✔ (docs-only change; ran for sanity)
• No new markdown directives, no new code samples; all internal links resolve against existing files.

Notes

• Follows the docs-sync style (no emdashes, no smart quotes).
• Branch is users/janechu/clarify-embedded-entry-in-integration-docs per my host repo's agent-driven branch convention.
• Replaces docs: add embedded fragment rendering recipe page #299. Independent of chore: correct rust integration docs to match published crate #295 / chore: correct node integration docs to match published render() signature #296 / chore: document WebUIHandler thread-safety #297 / feat: expose @microsoft/webui/platform subpath #298.