Switch to MKDocs + Material theme for documentation site

[madeleineostoja]

Gitbook toolchain (what we currently use) is essentially dead (GitbookIO/gitbook#1808 (comment)) and it's pretty ugly to boot. MDDocs looks like a promising alternative, and it has a lovely material theme that would fit well with Simpla's aesthetic. Should be mostly a drop-in replacement, since all our content is already organised logically.

[madeleineostoja]

There's also docsify which is an SPA built on Vue, looks pretty neat, and doesn't require Python like MKdocs