doc: use .adoc extension for AsciiDoc files

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.

It is much more common to use the ".adoc" extension for AsciiDoc files,
since this helps editors automatically detect files and also allows
various forges to provide rich (HTML-like) rendering.  Let's do that
here, renaming all of the files and updating the includes where
relevant.  Adjust the various build scripts and makefiles to use the new
extension as well.

Note that this should not result in any user-visible changes to the
documentation.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/trace2-target-values.adoc

@@ -0,0 +1,12 @@
+--
+* `0` or `false` - Disables the target.
+* `1` or `true` - Writes to `STDERR`.
+* `[2-9]` - Writes to the already opened file descriptor.
+* `<absolute-pathname>` - Writes to the file in append mode. If the target
+already exists and is a directory, the traces will be written to files (one
+per process) underneath the given directory.
+* `af_unix:[<socket-type>:]<absolute-pathname>` - Write to a
+Unix DomainSocket (on platforms that support them).  Socket
+type can be either `stream` or `dgram`; if omitted Git will
+try both.
+--