Do basic %var% templating for markdown files

Based on the answer here
gatsbyjs/gatsby#10174 (comment)

docs/dev-guide/service-preview.md

@@ -8,8 +8,8 @@ How do you verify that the code you've written actually works? Ambassador Pro's
 
 Download the latest version of the client:
 
-<a class="apictl-dl" href="https://s3.amazonaws.com/datawire-static-files/apictl/0.1.2/darwin/amd64/apictl">Mac 64-bit</a> |
-<a class="apictl-linux-dl" href="https://s3.amazonaws.com/datawire-static-files/apictl/0.1.2/linux/amd64/apictl">Linux 64-bit</a>
+<a class="apictl-dl" href="https://s3.amazonaws.com/datawire-static-files/apictl/%aproVersion%/darwin/amd64/apictl">Mac 64-bit</a> |
+<a class="apictl-linux-dl" href="https://s3.amazonaws.com/datawire-static-files/apictl/%aproVersion%/linux/amd64/apictl">Linux 64-bit</a>
 
 Make sure the client is somewhere on your PATH. In addition, place your license key in `~/.ambassador.key`.
 
@@ -67,7 +67,7 @@ In this quick start, we're going to preview a change we make to the QOTM service
              value: qotm
            - name: APPPORT
              value: "5000"
-           image: quay.io/datawire/ambassador-pro:app-sidecar-0.1.2
+           image: quay.io/datawire/ambassador-pro:app-sidecar-%aproVersion%
            name: traffic-sidecar
            ports:
            - containerPort: 9900
@@ -154,4 +154,4 @@ In this quick start, we're going to preview a change we make to the QOTM service
 
 Service Preview will match HTTP headers based on the headers that are seen by the *sidecar*, and not the edge gateway. Matches are made on the whole header, e.g., a match rule of `dev` will not match in the example above, while `/qotm/dev` will match.
 
-While any HTTP header will match, in practice, using host-based routing (i.e., the `:authority` header), a custom HTTP header (e.g., the `x-service-preview` header used above), or an authentication header is recommended.
+While any HTTP header will match, in practice, using host-based routing (i.e., the `:authority` header), a custom HTTP header (e.g., the `x-service-preview` header used above), or an authentication header is recommended.

versions.yaml

@@ -0,0 +1 @@
+aproVersion: 0.1.2