meta/new-web: add con.racket-lang.org

Slides from each year are kept elsewhere (much like the installers
for download.racket-lang.org).
meta/new-web: remove redirection of URLs for testing
rename "con" to "rcon"

The directory name "con" is not valid on Windows.
meta/new-web: streamline download sequence

Instead of requiring a trip through a page that lists mirrors, show a
direct link to the main-site download.

Also, make alternate distributions like "Racket Minimal" more
accessible.
Restyle mirror page.
meta/new-web: adjust one example to use `define` instead of `let*`
meta/new-web: try a different style for "explain" and "news"
meta/new-web: avoid "paradigms"
r{5,6}rs: fix `c[ad][ad]r` on wrong number of arguments

Closes PR 14388
create-embedding-executable: fix namespace and module-declaration problem
distro-build: split server and client parts
configure: propagate CPP, LD, CPPFLAGS, LDFLAGS for --enable-sdk build
subprocess: allocate for environment variables before fork

I'm running into stuck processes via `subprocess`, where the stuck
process is in malloc() for environment variables. I'm not really sure
of the problem, but moving that work before the fork() could
avoid problems related to pages or signal handlers.
changed introductory paragraph, used active voice in news, we need exact date
Update several application type error messages
Fix doc contract for `syntax-local-phase-level`

The description does not mention the function ever returning #f
and if you look at the implementation in the C code, it always
returns the result of `scheme_make_integer`.

Also add an example for the entry.
redex: add in-order enumeration to the benchmark

Also, add support for running a single file in parallel.
Fix TR tests for new error messages

Related to commit fa81fb35f7361f0a985ad4d311400d621a2a5496
Merge remote-tracking branch 'upstream/master'