docs: add features/app-install page for jss install #19

`jss install` shipped through Phases 1+2+4+6 of JSS#464. Substantial new feature with no docs yet.

Add `docs/features/app-install.md` alongside `git-integration.md` — install is a built-on-git feature, sits naturally as a feature reference rather than a guide. Update `sidebars.ts` to include it.

Page outline

1. Quick start — `jss install chrome`
2. App specs — bare name, `/`, full URL, `#` pinning, `=` rename
3. Auth — `--user` / `--password` / env, OR `--nostr-privkey` for NIP-98
4. Targeting — `--pod ` (default `http://localhost:4443\`)
5. Bundles — `--bundle `, JSON-LD shape, the `solid-apps/bundles` repo
6. How it works — git auto-init + dual-push under the hood
7. Troubleshooting — invalid spec, clone fail, auth fail, body-too-large (link JSS#474)

Companion

JSS-repo README will add a ~5-line section pointing at this page (separate PR).