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/RelNotes/1.5.0.6.adoc

@@ -0,0 +1,21 @@
+GIT v1.5.0.6 Release Notes
+==========================
+
+Fixes since v1.5.0.5
+--------------------
+
+* Bugfixes
+
+  - a handful small fixes to gitweb.
+
+  - build procedure for user-manual is fixed not to require locally
+    installed stylesheets.
+
+  - "git commit $paths" on paths whose earlier contents were
+    already updated in the index were failing out.
+
+* Documentation
+
+  - user-manual has better cross references.
+
+  - gitweb installation/deployment procedure is now documented.