patch 9.0.1001: classes are not documented or implemented yet

Problem:    Classes are not documented or implemented yet.
Solution:   Make the first steps at documenting Vim9 objects, classes and
            interfaces.  Make initial choices for the syntax.  Add a skeleton
            implementation.  Add "public" and "this" in the command table.

runtime/doc/Makefile

@@ -161,6 +161,7 @@ DOCS = \
 	version9.txt \
 	vi_diff.txt \
 	vim9.txt \
+	vim9class.txt \
 	visual.txt \
 	windows.txt \
 	workshop.txt
@@ -313,6 +314,7 @@ HTMLS = \
 	vi_diff.html \
 	vimindex.html \
 	vim9.html \
+	vim9class.html \
 	visual.html \
 	windows.html \
 	workshop.html