.gitignore: add built docs, sharness test results

Problem: Git does not ignore the built documentation under doc/_build
or the .log or .trs files as a result of running sharness tests.

Add the docs intermediate files and sharness test result files to the
list of ignored files by Git.

.gitignore

@@ -71,3 +71,11 @@ config.h.in
 config.h.in~
 config.h
 config.status
+
+# misc
+*.trs
+*.log
+
+# docs intermediate files
+/doc/man*/*.xml
+/doc/_build