doc: update gitignore for .adoc extension

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>