Allow linking to wiki page subsections by making headers link to the appropriate #markdown-header anchor

This seems to be the only thing that is done better in GitHub wikis. There, if you hover over a header in a wiki, there a paragraph character shows up, and if you click on the header, it updates the URL so that it is possible to send someone a link directly to a sub-section of a wiki page. In Bitbucket, we currently have to open the developer tools inspector, find the appropriate #markdown-header-something anchor, and copy-and-paste that into the URL.