Merge already existing mkdocs.yml config with techdocs-core config instead of override

[emmaindal]

Currently, the techdocs-core plugin overrides the markdown extension configs and set the new configs instead of taking the already existing configuration (in mkdocs.yml) and merge the two ones together.

This means if, for example I want to add toc_depth as configuration to my table of contents, I could add this to my mkdocs.yml file

markdown_extensions:
  - toc:
      toc_depth: 3

but this will be overridden by the configs set in the techdocs-core plugin.

Feature Suggestion

Merge the already existing config in the mkdocs.yml file with the config set in the techdocs-core plugin.