docs: remove extra images NOTE: history was mangled

This commit has two purposes. One thing it does is remove some small
unused image files that were part of leaflabs-docs relating to Maple
IDE.

The second thing it does is mark a point in git history where history
was rewritten permanently. A small number of larger images (totaling a
few MiB) were completely deleted from git history at this point in the
repository. These images existed in the leaflabs-docs side of history,
and would have bloated out the repository size from now to forever if
they hadn't been purged. An unfortunate side-effect of this is that now
the leaflabs-docs parent branch no longer matches the leaflabs-docs
upstream repository. Manually exported patches/diffs should still apply
just find in this repo.

The reduction in repository size was something like 5.4 MiB => 3.7 MiB
docs: several small theme tweaks

- CSS improvements
- two-part links at page top (project/subproject)
- "Project Resources" links
docs: clarify versioning
travis: specify language as cpp
docs: get webfonts from rambutan.cc instead of google
docs: fix header css

This got broken during a sloppy git rebase