Add a note on the path rewriting page to clarify that Pomerium does not modify upstream responses

[kenjenkins]

Page: https://www.pomerium.com/docs/reference/routes/path-rewriting

What's incorrect or missing

We've had a couple recent issue reports that path rewriting does not behave as users expect:

• Path rewriting not working as expected pomerium#5020
• Redirect with path variable not working pomerium#4909

As well as a similar Slack thread last month:

• https://pomerium-io.slack.com/archives/CK92MUAES/p1707343997402799

What's the resolution?

I think it would be good to add some note or callout on this page to mention that just rewriting a path prefix in Pomerium is often not enough to serve a particular upstream service under a path prefix. Many upstream services will also need to be configured appropriately, so that they add this path prefix to any link URLs, HTTP redirects, etc.

We could even add a few examples of relevant configuration options, to help users get a sense of what to look for when they search the documentation for the service they are using. For example:

• SCRIPT_NAME for pgAdmin 4
• QUERY_BASE_PATH for Jaeger