Publish public doc for Spring Boot 3 upgrade of a rules_spring service (e.g. OpenRewrite)

[plaird]

We have 100+ rules_spring.springboot() usages inside a Bazel monorepo internally. We have developed tooling for helping with the Boot2 -> Boot3 upgrade.

There are almost always manual fixes to do after the tooling runs, but it can solve most of the upgrade. We rely on a combination of:

• Docs with a lot of details
• OpenRewrite procedure (requires generating a pom.xml using pomgen)
• Various caveman bash scripts that invoke sed a lot

90% of this is not specific to Salesforce. We should provide this to other victims build teams.

Existing docs:

boot_launcher_class attribute change
The top level README explains the boot launcher change necessary.

Javax -> Jakarta package changes
We already have a focused javax->jakarta doc, but there is a lot more to this.