Restructure Spring documentation

[trask]

instrumentation/spring/README.md is all about non-javaagent usage, and could probably be structured better.

it has three parts:

• Manual Instrumentation with Java SDK
• Manual Instrumentation using Handlers and Filters
• Auto Instrumentation using Spring Starters

the first two parts aren't specific to any of the instrumentation modules in this repo, and just describe manually instrumenting Spring using OpenTelemetry SDK directly. These two sections may not belong in this repo at all.

the third part could then be moved to instrumentation/spring/starters

the only thing left in instrumentation/spring/README.md (for now) would be the settings section (and maybe a link to the starters doc).

[pizzaz93]

The first two sections would belong in https://github.com/open-telemetry/opentelemetry-java-docs or https://opentelemetry.io/docs/instrumentation/java/manual ?

The third section would be integrated into the starter sub folders or a general readme on that page?

[trask]

Closing, the Spring Boot starter docs have been completely re-written and now live on the website.