Documentation: Add documentation on jenkins builders

Put this in a new directory called 'infrastructure' and make a link
and an index.md file for the directory.

Signed-off-by: Martin Roth <martin@coreboot.org>
Change-Id: I54a0204e7525a25f2fd717a73007b304aac67396
Reviewed-on: https://review.coreboot.org/c/coreboot/+/43984
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

Documentation/infrastructure/index.md

@@ -0,0 +1,6 @@
+# coreboot infrastructure
+
+This section contains documentation about coreboot infrastructure
+
+## Jenkins builders and builds
+[Setting up Jenkins build machines](builders.md)