Document adding MkDocs after skipping docs tool

[Shizoqua]

Description

Closes #5603.

This PR documents how to add a MkDocs documentation site to a Kedro project that was created without selecting the docs tool.

Development notes

Updated docs/deploy/package_a_project.md to fill the previously empty section “Add documentation to a Kedro project if you have not selected docs tool”.

The new section:

• explains that Kedro’s generated docs tool scaffold is Sphinx-based
• gives MkDocs setup steps for a project without an existing docs/ directory
• adds mkdocs and mkdocstrings[python] dependencies
• shows how to create docs/index.md, docs/api.md, and mkdocs.yml
• includes API documentation from the generated project source with mkdocstrings

Verified with:

• uv pip install -e ".[docs]"
• .venv\Scripts\mkdocs.exe build --strict
• fresh kedro new --tools=none --example=n project, followed by the documented MkDocs setup and mkdocs build
• git diff --check -- docs/deploy/package_a_project.md

Vale and Lychee were not run because they are not installed locally.

Developer Certificate of Origin

Checklist

• Read the [contributing](https://github.com/kedro-org/kedro/blob/main/CONTRIBUTING.md) guidelines
• Signed off each commit with a [Developer Certificate of Origin (DCO)](https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/managing-repository-settings/the-commit-signoff-policy)
• Opened this PR as a 'Draft Pull Request' if it is work-in-progress
• Updated the documentation to reflect the code changes
• Added a description of this change in the [RELEASE.md](https://github.com/kedro-org/kedro/blob/main/RELEASE.md) file
• Added tests to cover my changes
• Checked if this change will affect Kedro-Viz, and if so, communicated that with the Viz team