docs: add VS Code and Zed editor setup guide (#22)

[meymchen]

Summary

Closes #22 — the VS Code / Zed MVP documentation work.

Adds an Editor setup section to the README so a user can install
lspf-hello and point VS Code or Zed at it without writing any code:

• Explains the Cargo workspace layout and that lspf-hello is an
  installable template server.
• Copy-paste VS Code settings.json snippet (via the Generic LSP
  Client extension) launching lspf-hello for plaintext.
• Copy-paste Zed settings.json snippet binding lspf-hello to the
  built-in Plain Text language.
• The cargo install --path crates/lspf-hello install command.
• A Troubleshooting subsection: binary not on PATH, server not
  starting, and reloading the editor window after editing settings.
• Fixes stale examples/hello references left over from the workspace
  migration ([VS Code/Zed MVP] Migrate repository to Cargo workspace #20/refactor: migrate repository to Cargo workspace (#20) #24).

Test-driven (doc-sync)

The seam is the copy-paste correctness of the snippets. Added
crates/lspf-hello/tests/editor_docs.rs (written red-first) which:

• parses the VS Code and Zed JSON snippets straight out of the README and
  fails if they aren't valid, paste-able JSON;
• asserts each snippet references the real lspf-hello binary and the
  plaintext / Plain Text language;
• asserts the install command and a Troubleshooting subsection are present.

This keeps the documented onboarding path from silently rotting if the
binary name or config shape ever changes.

Acceptance criteria

• README explains the workspace layout and that lspf-hello is an installable template server.
• Copy-paste VS Code settings.json snippet launching lspf-hello for plain text.
• Copy-paste Zed configuration snippet launching lspf-hello.
• Install command cargo install --path crates/lspf-hello.
• Troubleshooting section for common issues.
• Manual validation in Zed — left for the maintainer (cannot be automated here; the doc-sync test guards the snippet contents).

Verification

• cargo test --all-features --all-targets — all pass (new editor_docs doc-sync tests + existing smoke e2e tests).
• cargo clippy --all-targets --all-features -D warnings — clean.
• New files are LF / rustfmt-clean.

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