Overhaul README.md in this repository

[sxa]

There have been very few updates to the top level README.md in the temurin-build and ci-jenkins-pipelines repositories, and so we should look at them and see if they are "fit for purpose" and whether the information on here that we present to our user base is (a) useful (b) accurate and (c) fit for purpose.

I've started with an overhaul of the ci-jenkins-pipelines README.md in adoptium/ci-jenkins-pipelines#1048 but here are some discussion points for the one in here:

• We have multiple ways of setting up a build environment - what do we want to recommend and promote?
  • Run the playbooks on your local machine
  • List the prereqs that are required (Maybe complex as it's different on each platform?)
  • Use the -D option on makejdk-any-platform.sh to automatically build in a new container image to avoid discouraging podman users (Also, why do we have this section yet also reference the dockerfile generator in the "build natively" section? Is this likely to be confusing to first time users?) NOTE: This is converted to -D in https://github.com/adoptium/temurin-build/pull/3796/files#diff-b335630551682c19a781afebcf4d07bf978fb1f8ac04c6bf87428ed5106870f5
  • Should we recommend the use of our adoptopenjdk/*_build_image containers?
  • With a more portable devkit on the more common Linux platforms, perhaps we can significantly simplify the process for JDK21+ and encourage people to go down the "bring their own OS" and use that with the devkit?
• Duplicating the "man page" for makejdk-any-platform.sh in the README.md seems unnecessary - we should only have this information in one place ... But which location is right? Is the "man page" format still useful? Have we ended up with updates in there that aren't in the README.md?
• Is the metadata section useful? Should it be in this repository's README or the ci-jenkins-pipelines one? My gut feel is that it should probably be here, but maybe collapsed (as I did in my pipelines PR for now) or moved elsewhere as it's perhaps not "interesting" particularly now that we have the SBoM generation.
• We should perhaps shout more about SBoM and reproducibility options in the README.md which is a significant change in the last two years.
• Should SmokeTesting.md and Testing.md be moved somewhere else instead of the top level directory? Maybe Testing.md should be part of CONTRIBUTING.md and the smoke test doc to test/functional/buildAndPackage/README.md?
• There are references to the docs directory in this repository which no longer exists which should be updated or removed.

Overall we should also try and tidy up our recommendations on where things affecting the builds should be changed e.g. configure-args etc. since we do have multiple places where these can be set. (There's some info on this, but more discussion would be useful)