Add project: mkdocs-unirate

[rob-browncc]

Adds mkdocs-unirate to the code-exec-templating category (variables & templating).

mkdocs-unirate is an MkDocs plugin that injects live UniRate currency exchange rates into docs at build time, alongside plugins like macros and markdownextradata:

• Inline Markdown placeholders: {{ unirate_rate:USD:EUR }} and {{ unirate_convert:USD:EUR:100 }}
• A Jinja unirate global for themes/templates ({{ unirate.rate('USD','EUR') }}, {{ unirate.currencies }})
• strict mode aborts the build on failure; non-strict logs a warning and leaves the placeholder in place
• Runtime dependencies: mkdocs + requests only

Entry added to projects.yaml (never the README, per the contribution guidelines):

- name: mkdocs-unirate
  mkdocs_plugin: unirate
  github_id: UniRate-API/mkdocs-unirate
  pypi_id: mkdocs-unirate
  labels: [plugin]
  category: code-exec-templating

GitHub: https://github.com/UniRate-API/mkdocs-unirate · PyPI: mkdocs-unirate (MIT).

[rob-browncc]

Heads-up: the failing CI / Check projects run here isn't from this addition — it's the pre-existing Zettelkasten entry-point mismatch (Missing entry point [mkdocs.themes] 'zettelkasten-solarized-light'), which is also red on main right now. This PR only adds a single mkdocs-unirate entry under code-exec-templating; check_projects.py doesn't flag it. Happy to adjust the category or labels if you'd prefer it elsewhere.