Move from .md to .markdown for JJB docs

As [explained in detail][0] on the integration-dev mailing list, there
are good reasons to use .markdown for Markdown-formated docs.

Default-config Vim's syntax highlighting rules interpret this file
as Modula 2 source code with .md, Markdown with .markdown.

[0]: https://lists.opendaylight.org/pipermail/integration-dev/2015-
November/005113.html

Change-Id: I0495912c377577a5f4c0669c9d9df3a5aab86666
Signed-off-by: Daniel Farrell <dfarrell@redhat.com>

diff --git a/src/site/markdown/jenkins.md b/src/site/markdown/jenkins.markdown
similarity index 100%
rename from src/site/markdown/jenkins.md
rename to src/site/markdown/jenkins.markdown