doc: make it easier to read how to enable extensions We tell people all the time that enabling extensions is not a scary thing to do, but we don't make it easy enough for an absolute novice to do so. When they see a suggestion to do "hg extfoo bar", the error message tells them "see hg help extensions", but that help page doesn't actually tell them where configuration files are. Furthermore, the big warning about why extensions aren't enabled by default should be pushed down a little bit. Most of the extensions shipped by hg are not all that scary, and some very basic and useful cosmetic extensions like graphlog, color, pager, and progress, should be enabled for many hg users.