ardupilot_wiki duplicated

[fnoop]

~/var/lib/web/ardupilot_wiki ~/software/ardupilot_wiki

[fnoop]

ok this is because the repo is cloned to ~/software and then compiled/built and installed to ~/var/lib/web. Each directory ends up as about 450mb, mostly due to ludicrously large images. The size of distributing this by default outweighs the benefits, so:

• [x] Turn off install/build by default
• [ ] Clone to ~/var/build by default, but make it configurable. If user wants to contribute to docs then this could be changed to ~/code.
• [ ] Install to ~/software, rather than ~/var/lib/web. ~/var should only be for transient/temporary files.

[fnoop]

Related to #981 - also turn off px4 docs by default?