Delete gh-pages profile from titan-site

I used this profile to experiment with the Github maven plugin, but it
proved too destructive in testing to use in actual releases.  It
doesn't appear to offer an easy way to replace a subdirectory with a
new one a la rsync, so its tricky bordering on impractical to use it
to modify subdirectories of gh-pages's javadoc and wikidoc
directories.  All its prospective titan-site functionality has been
subsumed in the gh-pages-update.sh script anyway.
Update gh-pages index.html template
Remove symlinking from gh-pages-update

gh-pages does not support symlinks and returns 404s on them
Update outdated titan-dist/README.md
Document Titan release process RELEASING.md
Make rewrite-tag.sh more robust

This script now reads the current HEAD branch using `git symbolic-ref`
and attempts to return to that HEAD after rewriting the release tag.
Release instructions update