Make struct: work at the REPL

Closes PR 11669
add a section to the image guide about rotation and centers
make doc-search URL configurable for installer builds

Also, add an initial-catalogs configuration to clients and
`farm' builds.
Generalize use of farm config file

Change `FARM_CONFIG' to just `CONFIG' and use it on the server, too.
racket/sandbox: repair for link search paths
allow any real number (except +inf.0, -inf.0, and +nan.0)
as angles

also bring image.scrbl down below 102 columns
clarify wording
Change "farm" terminology to "site", mostly
fix "distro-build"
another "distro-build" fix
distro-build/assemble-site: merge native libraries
Support require/typed clauses in require/typed/provide

Closes PR 13901
Fix reading of config.rktd file when in strange readtable.
Examples for #:opaque and require/typed/provide
Move json tests to racket-test pkg.
Don't run special sandbox tests when the relevant libs are not available.
Create `unstable-debug-lib`, `unstable-pretty-lib`, `unstable-flonum-lib` packages.
Add `scribble-text-lib` package.
Add more correct dependencies for `racket-test`.
Fix unneeded R6RS dependency.