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>

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