Added plugin config definition

[DSmedley]

Added a plugin config definition as defined in the mkdocs dev-guide for developing a plugin. This resolves the issue where mkdocs will throw a warning when any property is passed to the techdocs-core config.

Example error when running techdocs-cli serve -v:

verbose: [mkdocs] WARNING -  Config value 'plugins': Plugin 'techdocs-core' option 'use_material_search': Unrecognised configuration name: use_material_search
verbose: [mkdocs] WARNING -  Config value 'plugins': Plugin 'techdocs-core' option 'use_pymdownx_blocks': Unrecognised configuration name: use_pymdownx_blocks

This was ran with the following plugin configurations:

plugins:
  - techdocs-core:
      use_pymdownx_blocks: true
      use_material_search: true

[dlaird-ovo]

Might need help from @awanlin 👍