Simplify and fix doc-related .gitignore files.

Simplify and fix doc-related .gitignore files.

This fixes some unwanted 'git status' output after 'make docs'.
* .gitignore: Add **/Makefile, **/makefile.
Remove Makefile, makefile.
Add doc/*/*.xxx rules for the usual kinds of documentation outputs
and temporaries.  Add doc/misc/cc-mode.ss.
* doc/emacs/.gitignore, doc/lispref/.gitignore, doc/misc/.gitignore:
* lwlib/.gitignore, oldXMenu/.gitignore:
Remove, as these .gitignore files are no longer needed.
* doc/lispintro/.gitignore: Replace with list of exceptional PDF files.