doc: update gitignore for .adoc extension

We presently use the ".txt" extension for our AsciiDoc files. While not wrong, most editors do not associate this extension with AsciiDoc, meaning that contributors don't get automatic editor functionality that could be useful, such as syntax highlighting and prose linting. Instead, in a future commit, we're going to move to using the more common ".adoc" extension for these files, which many editors intrinsically recognize as an AsciiDoc file. To avoid contributors accidentally checking in generated files, ignore the new extension for generated files in the documentation .gitignore files. Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>