docs: activate mermaid dark theme for hexdocs

[clsource]

Improved hexdocs rendering

[clsource]

This PR solves two problems in Hexdocs

1. Does not render diagrams in architecture

This is due to the odd code used for triggering renders.
It was used a standard code for rendering all .mermaid code blocks.

2. Added dark theme

The current white theme does not look good in hexdocs dark mode.
Maybe it can be improved to be the same as the hexdocs option.