Clear up the wiki in this repository

[sxa]

There are quite a few pages which look like they may be obsolete or otherwise do with an overhaul. Some of them can probably be moved into the new build FAQ #1457 - but we should revisit it all.

[sxa]

@karianna Ref the other pages we have:

• Other OpenJDK build efforts to integrate including ojdkbuid nad David Lloyd's OpenJDK - I don't believe we've properly looked at either of these so is it still in the plan?
• Difference between adopt and oracle builds which is probably best superceded by the migration guide to avoid information fragmentation
• Getting started which talks about makejdk.sh which no longer exists in this repo and should be superceded by the README.md in here
• OpenJDK11 release status which as @smlambert says is deprecated and rather pointless given it's only for one release
• The [OpenJ9 guide] may still be useful, although most info related to that is now in the RELEASING.md