Using a Makefile instead of docker-compose.yml for doc-en

Hi,

I've created a Makefile for the `doc-en` repository that runs docker directly for doing smoke-testing/building. It will use an adjacent `doc-base` and/or `phd` if it finds them, which  was my original motivation. There's also a `php` target if you want to build the web version of the manual.

https://github.com/php/doc-en/pull/3673

With this, instead of running `docker-compose up` to build, you'll just need to run `make`. (The README.md has been updated.)

I'll push it next week unless there are objections here.

There's also a pending PR for the `phd` repo that adds a Makefile for running the test suite there:

https://github.com/php/phd/pull/147

Thanks.

Jim