setup/[main-]collects: doc and error-message repairs

Closes PR 14222
scribble: doc repair

Closes PR 14219
racket/enter: add `dynamic-enter!`
scribble/manual: make `racketblock` fall back to `racket`-style spacing

When source-location information isn't available, usually due to macros
in compiled modules, make `racketblock` fall back to `racket`-like
spacing. This improvement is visible, for example, in the documentation
for functions like `recycle-icon`.

Using `quote-syntax/srcloc` in a macro is probably a better solution,
because that will preserve the original layout including line breaks,
but the default-spacing fallback seems like an improvement over
unreadable output.
scribblings config: 'keep-style and 'no-search flag, 'omit-start section

The 'keep-style avoids imposing the Racket manual style on a
document, while 'no-search omits the search box. The 'omit-start
section is like 'omit, but the document is still indexed and
available for cross-references.

Also, recognize a 'prefixable tag shape so that the set of tags
hat support prefixing is extensible.
scribble: document all `scribble/base` CSS style classes

A demo document (linked from the Scribble manual) demonstrates how
to trigger each style class and what it looks like with the default
and manual styles.
add some missing custom printers (to recursive-contract)
strengthen shortcut-prefix contract

(technically, strengthen the way it is written in the docs; it
was already being enforced that way)
net/http-client: restore use of `abandon`

The difference between `abandon` and `close-output-port` seems to
matter for downloading PLaneT packages on some networks. Using
`abandon` does not seem to create any problems now that an extra
"\r\n" is not incorrecty included in PUT/POST requests.
scribble: document all `scribble/manual` CSS style classes

Includes a few Scrible bug and documentation repairs.
racket/gui win32: fix for backing-scale

Closes PR 14278