Add assorted other documentation build targets to documentation gitignore.

doc/src/sgml/.gitignore

-/html
+# Stuff shipped in tarballs
+/html/
 /html-stamp
-/man1
-/man3
-/man7
+/man1/
+/man3/
+/man7/
 /man-stamp
-
+# Other popular build targets
+/HISTORY
+/INSTALL
+/regress_README
+/postgres-US.pdf
+/postgres-A4.pdf
+/postgres.html
+/postgres.txt
+# GENERATED_SGML
 /features-supported.sgml
 /features-unsupported.sgml
 /version.sgml
+/bookindex.sgml
+/HTML.index
+# Assorted byproducts from building the above
+/postgres.xml
+/HISTORY.html
+/INSTALL.html
+/regress_README.html
+/postgres-US.aux
+/postgres-US.log
+/postgres-US.out
+/postgres-A4.aux
+/postgres-A4.log
+/postgres-A4.out