Replace digest-doc and sorted-doc C programs with Lisp commands.

Replace digest-doc and sorted-doc C programs with Lisp commands.

* lib-src/digest-doc.c, lib-src/sorted-doc.c: Remove files.
* lib-src/Makefile.in (UTILITIES): Remove digest-doc and sorted-doc.
(digest-doc${EXEEXT}, sorted-doc${EXEEXT}): Remove rules.
* lib-src/makefile.w32-in (ALL): Remove digest-doc and sorted-doc.
($(BLD)/sorted-doc.exe, $(BLD)/digest-doc.exe, sorted-doc, digest-doc)
($(BLD)/digest-doc.$(O), $(BLD)/sorted-doc.$(O)): Remove rules.
(install): Don't install digest-doc.exe or sorted-doc.exe.

* lisp/help-fns.el (doc-file-to-man, doc-file-to-info): New commands.

* doc/lispref/help.texi (Documentation Basics): Remove mentions of
digest-doc and sorted-doc.

* INSTALL, nt/README, nt/README.W32: Do not mention digest-doc and sorted-doc.

* etc/NEWS: Mention this change.