Add existing XenAPI-related docs

These were extracted from the PDF version of the XenAPI reference as well as
the SDK guide.

Signed-off-by: Rob Hoes <[email protected]>

Author: robhoes

_data/navbar.yml

@@ -14,9 +14,15 @@
   - features/backtraces.md
 - title: XenAPI
   docs:
-  - xen-api/index.html
+  - xen-api/overview.md
+  - xen-api/usage.md
+  - xen-api/http.md
+  - xen-api/extensions.md
+  - xen-api/wire-protocol.md
   - xen-api/evolution.md
+  - xen-api/vm-lifecycle.md
   - xen-api/udhcp.md
+  - xen-api/index.html
 - title: Xapi
   docs:
   - xapi/overview.md