This PR removes the old documentation overview site, as its content has
moved to the main Lean website infrastructure.
This should be merged when the new website section is deployed, after
installing appropriate redirects.
Developer documentation is remaining in Markdown form, but it will no
longer be part of the documentation hosted on the Lean website. Example
code stays here for CI, but it is now rendered via a Verso plugin.
This PR clarifies the styling of `do` blocks, and enhanes the naming
conventions with information about the `ext` and `mono` name components
as well as advice about primed names and naming of simp sets.
