Remove wildcard rule from .gitignore file.

The "*.*" rule in the .gitignore file effectively means no untracked
files are ever shown. This makes it too easy to forget to add new files.

This commit also explicitly ignores anything under bin.v2/, which is
where Boost.Build places all build artifacts by default.
Re-add documentation, test cases and root files.

These files were erroneously missing when moving files around. Commit
e9faaeee fixes the cause of these missing files not being spotted, which
shows up all of the files that need to be git add'd.