Upgrade doc dependencies and update usage of mkdocs

[oprypin]

• Use H1 top-level headings, not H2
  • This integrates nicer with the ToC on the right, but also forces all headings to become larger
  • Some styles had to be dropped or modified due to this
• Remove some styles that mkdocstrings-crystal itself guarantees
• Simplify the type-walking script, a lot was redundant

Extras:

• Nicer nav via https://squidfunk.github.io/mkdocs-material/setup/setting-up-navigation/#navigation-sections
• Include the changelog into the site via https://facelessuser.github.io/pymdown-extensions/extensions/snippets/#formatting-snippets

[oprypin]

(Please check it out locally before merging; check whether the shift in headings and the restyle is acceptable)