Document/improve Tilt setup with local version of apm-data

When working on a local version of apm-data, as documented in the [apm-data documentation](https://github.com/elastic/apm-data/blob/main/dev_docs/HOW_TO.md#2-make-changes-in-apm-server), we need to make the build use a local copy of apm-data with a command similar to:

```
go mod edit -replace=github.com/elastic/apm-data=/path/to/your/apm-data
```

While this works well for a local build of `apm-server`, this won't work with Tilt setup which provides a very convenient end-to-end setup and makes every agent developer a "10x engineer".
Because the `apm-data` build happens within Docker, we need to have `/path/to/your/apm-data` to be within a subfolder of the `apm-server` repository.

In order to make this work, I had to do the following:
- within `apm-server` root folder, clone a copy of `apm-data`
- add `replace github.com/elastic/apm-data => ./apm-data` to `go.mod` (equivalent to the `go mod edit -replace` above)
- add `COPY apm-data/ /src/apm-data` just after `COPY go.mod go.sum /src/` within `packaging/docker/Dockerfile`

I think we should attempt to address the following points, and fallback to document this
- the `apm-data` repository is often cloned next to `apm-server`, not as a direct subfolder (minor, but could be annoying)
- we need to modify the Dockerfile to make this work, otherwise the build will fail as the dockerized build can't access a folder above `apm-server`

Read the apm-data HOW_TO.md section linked in the issue, then inspect go.mod and packaging/docker/Dockerfile alongside the local Tilt setup. Compare support for an apm-data checkout nested under apm-server with the documented sibling checkout, and verify the end-to-end Dockerized build. Done means either the local setup works without manual Dockerfile changes or the fallback procedure is documented clearly.

Written by the indexing model from the issue text.