docs(install): add pipx as alternative installation method (#2288)

* docs(install): add pipx as alternative installation method

- Add pipx commands to README.md installation section
- Add note about pipx compatibility to docs/installation.md
- Mention pipx persistent installation in docs/quickstart.md
- Add pipx upgrade instructions to docs/upgrade.md
- Clarify that project has no uv-specific dependencies

Refs: https://github.com/github/spec-kit/discussions/2255

* docs(install): address Copilot feedback - update prerequisites and upgrade references for pipx

* Update docs/quickstart.md

markdownlint’s MD012 (enabled in this repo) flags multiple consecutive blank lines

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

* Update docs/upgrade.md

In the Quick Reference table, the label “pipx upgrade” is misleading because the command shown is `pipx install --force ...` (a reinstall). by copilot.

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <175728472+Copilot@users.noreply.github.com>

docs/quickstart.md

@@ -22,6 +22,17 @@ uvx --from git+https://github.com/github/spec-kit.git specify init <PROJECT_NAME
 uvx --from git+https://github.com/github/spec-kit.git specify init .
 ```
 
+> [!NOTE]
+> You can also install the CLI persistently with `pipx`:
+> ```bash
+> pipx install git+https://github.com/github/spec-kit.git
+> ```
+> After installing with `pipx`, run `specify` directly instead of `uvx --from ... specify`, for example:
+> ```bash
+> specify init <PROJECT_NAME>
+> specify init .
+> ```
+
 Pick script type explicitly (optional):
 
 ```bash