No built docs

### Remove all built files

Files that are built do not belong in a repository. These are
appropriate for a release tarball but should not be included in a
repository. Keeping built files around causes too much churn, such as
that in #185, where the author had a different version of docutils,
which caused every built file to change.

Issue #185 should wait for this to go through and rebase to avoid the
unnecessary churn.


### Move built files to a dedicated directory

It was unclear previously which files were generated and which were
one-offs that are independent of the TEP building process. Worse, the
overzealous clean rule would delete files such as `html/install-tinyos.html`
inappropriately.