Feature: Better theme for default template

[ShukantPal]

@webdoc/default-template now uses system fonts, larger fonts, and more whitespace. This should make it more pleasurable to read.

This work also adds support for mermaid-js diagrams. See more details about how it works at: https://mermaid-js.github.io/mermaid/#/

Some other bugs have also been fixed:

• {@link } also works with tutorials now.
• The webdoc watermark ("Powered by webdoc!") has moved out of the footer and in its own element. The footer can be customized without removing the watermark now.