First cut at handling `.scrbl` sources, too.
Don't be sloppy and rush to commit something after a rename.
Use ~> don't need ~>>
Use simpler and more tolerant match for Scribble content.
Better approach to Scribble CSS.

- Add/use css/scribble.css. Note: The contents are more like
  racket.css in typical Scribbling -- for <span> elements like syntax
  highlighting. We don't use the whole page generated by Scribble, so
  block elements and things like table of contents are N/A.

- Don't add Pygments classes, too. Just go with the "RktXxx" Scribble
  classes.

- Do continue to add newlines (for the <pre> blocks), and may as well
  simplify some HTML like that for spaces as well as removing any
  empty RktMeta elements.
Hoist headings up 2 levels not 1.
Improve comments and add @interaction example.
Make "sCodeFlow"s a bit more like BootStrap default theme.
Much simpler strategy: Just change SCodeFlow blockquote to div.
Link to documentation from Racket code blocks.

Having added support for Scribble files, Markdown is envious that
Scribble code blocks can link to documentation.

In Scribble, you get the links as a result of doing a
`(require (for-label ...))` of the module(s) you want to be used. One
the one hand it's annoying when you forget to do this. On the other
hand, when a function is provided by multiple modules --
e.g. `rectangle` or `printf` -- you control _which_ lib's
documentation is linked to.

In Markdown, you don't do this. Instead `doc-uri.rkt` has a heuristic
to pick the lib automatically. If provided by exactly one lib, of
course use that. If provided by two or more, and one of them is racket
or racket/base, use that (handles e.g. `printf`). Otherwise don't link
at all (the fate of e.g. `rectangle`).
Doc links should use "color: inherit" style.

That way, they look like the color for the type of syntax element they
are, not the color of a link.
Various changes:

- Refactor code.

- In the case where a symbol is provided by multiple modules, doc-uri
  returns a list of modules and URIs (instead of a list of
  modules). That way, a caller could provide a list of the choices for
  a user.

- Ignore Planet modules: They make xref-tag->path+anchor blow up
  because they're not something path-element->string can handle.

- Handle xref-binding->definition-tag returning #f.
Just style edit: Square brackets for attributes.
Delete some debugging `displayln`s.
Update unit test for previous commit.
Refactor into cleaner set of functions.

Also optimize: Build and use a hasheq cache from name to libs (instead
of repeatedly scanning the 27,000+ items in the entry? list, as in the
original code I'd cribbed from help/help-utils).
Refactor.
Various changes:

- Add and use `xexpr-map` for higher-order xexpr transforms.

- Move some code to new util.rkt.

- Support images generated by Scribble.

- Add example of a non-post page sourced in Scribble.
Use memoization for `doc-uri`. Also update/simplify unit tests.
Added copying of Scribble generated gif and svg files