Add how-to for ClientSettingsPolicy

[kate-osborn]

Proposed changes

Problem: No user guide on how to use ClientSettingsPolicy.

Solution: Add a how-to guide that explains how to use ClientSettingsPolicy to configure client max body size.

Testing: Ran through steps and made sure they worked

Please focus on: Is it enough to show how to configure client max body size? Should we show off other client settings? Client max body size is the best demo end-to-end. We could set keepalive requests to 0 and show that the connection is not reused in the curl logs, but that isn't as impactful.

Closes #1763

Checklist

Before creating a PR, run through this checklist and mark each as complete.

• I have read the CONTRIBUTING doc
• I have added tests that prove my fix is effective or that my feature works
• I have checked that all unit tests pass after adding my changes
• I have updated necessary documentation
• I have rebased my branch onto main
• I will ensure my PR is targeting the main branch and pulling from my branch from my own fork

Release notes

If this PR introduces a change that affects users and needs to be mentioned in the release notes,
please add a brief note that summarizes the change.

NONE

[pleshakov]

looks good to me

Please focus on: Is it enough to show how to configure client max body size? Should we show off other client settings? Client max body size is the best demo end-to-end. We could set keepalive requests to 0 and show that the connection is not reused in the curl logs, but that isn't as impactful.

just max body size is 👍

[kate-osborn]

I'm still working on the doc on policy merging and inheritance behavior (#1995), but this can be reviewed and merged separately, so marking as ready.

[ADubhlaoich]

LGTM, made a bunch of suggestions to add a declarative, imperative tone.

We avoid passive voice where possible and don't want to be too familiar, which is why I got rid of all of the "We" and "Let's" I spotted.

[kate-osborn]

@ADubhlaoich I re-requested your review because I added a couple callouts about a known limitation: d341771

[bjee19]

lgtm 👍

[ADubhlaoich]

LGTM still, small suggestion for phrasing the tracing note.