DOCS: Logging into the web client

Adding a login section to the web client docs.

Signed-off-by: Kathy Lussier <klussier@masslnc.org>
DOCS: Tab documentation for web client

Replacing the XUL-based tab docs with general tips for using web browser
controls, shortcuts, add-ons to support navigation in the web client.

Signed-off-by: Kathy Lussier <klussier@masslnc.org>