Update version of Vagrant from 1.8.6 to 2.0.2.
Update version of VirtualBox from 5.1.8 to 5.2.6.
We needed to update Vagrant because older versions have networking
problems on modern Macs, resulting in weird `apt` errors when
provisioning.
Commit message and some text tweaked by tabbott.
This commit helps reduce clutter on the navigation sidebar.
Creates new directories and moves relevant files into them.
Modifies index.rst, symlinks, and image paths accordingly.
This commit also enables expandable/collapsible navigation items,
renames files in docs/development and docs/production,
modifies /tools/test-documentation so that it overrides a theme setting,
Also updates links to other docs, file paths in the codebase that point
to developer documents, and files that should be excluded from lint tests.
Note that this commit does not update direct links to
zulip.readthedocs.io in the codebase; those will be resolved in an
upcoming follow-up commit (it'll be easier to verify all the links
once this is merged and ReadTheDocs is updated).
Fixes#5265.
Aman Agrawal
Rohitt Vashishtha
Harshit Bansal
Tommy Ip
David Rosa Tamsen
Tim Abbott
derAnfaenger
Sarah