Clean up .gitignore

Several files were removed from the doc sources and config, so there are a
few folders in .gitignore, which are not needed anymore. Also there was a
build directory referenced and the only build directory is under the doc
folder, so this was corrected also.

Change-Id: I60f25d282caa813b3bfc790988578b9882ba95ab
Merge "Clean up .gitignore"