Jenkins add htmlproofer

[dagar]

@hamishwillee can you review what you want here and we'll get it added to Jenkins.

-- htmlproofer ./_book --empty-alt-ignore true --check-external-hash true --http-status-ignore 503 --url-ignore 'https://img.shields.io/badge/discuss-px4-ff69b4.svg,https://img.shields.io/github/release/PX4/Firmware.svg,https://ninja-build.org/,https://github.com/torvalds/subsurface/blob/a48494d2fbed58c751e9b7e8fbff88582f9b2d02/README#L88-L115,https://download.libsodium.org/doc/,https://www.alfa.com.tw/products_show.php?pc=67&ps=241,http://www.qt.io/download-open-source/#section-6,http://gazebosim.org/tutorials?tut=ros_wrapper_versions#Gazebo7.xseries,https://github.com/PX4/Firmware/blob/master/launch/multi_uav_mavros_sitl.launch#L21-L28,https://github.com/PX4/Firmware/blob/master/launch/multi_uav_mavros_sitl.launch#L85-L93,https://support.intrinsyc.com/attachments/download/1515/Flight_3.1.3_qrlSDK.tgz'

[hamishwillee]

The text used for travis should be fine for jenkins. Note that this should not fail the build. Bad links happen all the time, and may not be related to any particular commit. I do however need to be able to check this log periodically so I can go through and fix up links.