Include empty symbol in variable enumeration

Please merge into 6.0.
Initial version of enumeration docs

(Please include on the release branch --robby)
Misc redex enumerator tweaks:

- edits to the docs
- adjust the #:i-th argument interpretation to allow
  any natural number (using modulo for finite enumerations)
- remove unnecessary gui dependency of a few test files

Please include on the release branch
Fix Scribble doc typos

Please merge to v6.0
Fix PR14207, thanks ryanc
misc small doc typos

please include on the release branch
followup fix that should probably have been in a7c6dfa05
some white-on-black color improvements for drracket

please include on the release branch
fix cons/c use in contract

related to PR 14222

This doesn't completely fix that PR because the
implementation says that bytes are allowed and
the docs don't allow that. My guess is that the
implementation is wrong, not the docs, btu I'm not
sure and this may be a backwards compatibility
kind of thing, too. Also, path->collects-relative
should probably be considered (it doesn't allow
bytes? as an argument).
small Scribble CSS fixes
add missing weight of Source Code font
raco: skip problem "info.rkt" files

Allows some `raco` tools to work (such as `raco pkg`) when loading
information about other installed tools fails, so that problems can
be more easily corrected using working tools (other than `raco setup`,
which is a special case, anyway, for bootstrapping).

Closes PR 14221

Merge to v6.0
games: fix "About Games" menu item

Merge to v6.0
minor doc repair (sentence starting with non-capitalized name)

Merge to v6.0
scriblib/fgure: hyperlink figure references in PDF

Merge to v6.0
Add remaining racket/string types

Please merge to v6.0
Add type for `regexp-replaces`

Please merge to v6.0
scriblib/figure: fix figures as ref targets

Merge to v6.0
Fix doc typo.

Closes PR14209.
Fix type expansion suggestion.

Closes PR14212.