Rename to govuk-mkdocs-material

[gwionap]

Changes necessary for renaming. These include:

• Updated structure with package under src directory
• Updated references to MkDocs Tech Docs Template in site files to GOVUK MkDocs Material
• Updated repo urls in anticipation of change
• Updated package config file to find package under src
• Updated test pypi action to trigger on changes to src/govuk_mkdocs_material/__init__.py

Will bump package version once agreed these changes look ok.

Note: Package built successfully locally.