build: use mkdocs-monorepo-plugin to include sub-projects

[tumidi]

1) Integrates sub-projects using mkdocs-monorepo-plugin

• Build process stays the same for production (using git submodules).
• common, sdk, server now have their own mkdocs.yml that contains the nav for their sub-pages (example: questionpy-common)

2) Enables us to build the docs from local dev repositories

• Introduces mkdocs-devbuild-plugin that patches the configuration on-the-fly.
• Enables the docs:build/docs:watch tasks in qpy-dev.

Note: Updated mkdocs navigation in separate PR #6.