minor db doc improvements
updated example paths

Closes PR 13995.
updated command line in docs

Closes PR 13996.
This file is only needed for `file/gif`.
Use `case` instead of `cond`
add immutable hash constructors and selectors to #lang info
clean up string-constants expansion and add dynamic-string-constant[s]

This saves about 200 bytes per use of (string-constant ...) in the .zo file.

Also, it now only requires a single string-constants file in phase 1
(instead of all 13) so that should be a savings of memory and time
when compiling .zos, too.
Add type for `this-language` to fix test

(also add a type for `all-languages`)
Fix indentation
fix string-constants
Add type for `dynamic-string-constants`
Adjust test for API coverage

Also Rackety
fix bug in DrRacket
raco pkg create: extend `--from-install` mode so it can unmove files

This adjustment to packing allows a native-library package to
be converted back from installed to installable.
raco setup: add `install-platform` to "info.rkt"

The `install-platform` value determines whether files are
copied to "lib" and "share" directories.
raco pkg {install,update,migrate}: add `--all-platforms`

The `--all-platforms` flag causes the package manager to follow all
package dependencies, even for dependencies that are specific to
platforms other than the current one.
declare platform for each native-library package
makefile fix for catalog-based installer build
remove bad `return`
make catalog-based installer build work

A catalog-based installer build (as opposed to an installer build
that uses "pkgs" and "native-pkgs") relies on the new `--all-platforms`
package-install flag and related support for re-packing installed
platform-specific packages.