What's new and migrating are prominently featured

Before this commit those were burried in the Appendix where few would
guess to look. Previously that information was at the top, so it makes
sense to point to the Github wiki from the index HTML page.

Also a general update of the index page. Since we have recently revised
the overview, we can simply point to it and keep it shorter.

The section listing now includes more details per section.

src/docs/asciidoc/overview.adoc

@@ -1,6 +1,7 @@
 :toc: left
 :toclevels: 1
 :docinfo1:
+
 = Spring Framework Overview
 
 Spring makes it easy to create Java enterprise applications. It provides everything you
@@ -19,6 +20,7 @@ Spring is open source. It has a large and active community that provides continu
 based on a diverse range of real-world use cases. This has helped Spring to successfully
 evolve over a very long time.
 
+[[overview-spring]]
 == What We Mean by "Spring"
 
 The term "Spring" means different things in different contexts. It can be used to refer to
@@ -41,6 +43,7 @@ A note about modules: Spring's framework jars allow for deployment to JDK 9's mo
 the same naming pattern with "-" instead of ".", e.g. "spring-core" and "spring-context").
 Of course, Spring's framework jars keep working fine on the classpath on both JDK 8 and 9.
 
+[[overview-history]]
 == History of Spring and the Spring Framework
 
 Spring came into being in 2003 as a response to the complexity of the early
@@ -81,6 +84,7 @@ among others. It’s important to remember that each project has its own source
 issue tracker, and release cadence. See https://spring.io/projects[spring.io/projects] for
 the complete list of Spring projects.
 
+[[overview-philosophy]]
 == Design Philosophy
 
 When you learn about a framework, it’s important to know not only what it does but what
@@ -103,6 +107,7 @@ that are intuitive and that hold up across many versions and many years.
 meaningful, current, and accurate Javadoc. It is one of very few projects that can claim
 clean code structure with  no circular dependencies between packages.
 
+[[overview-feedback]]
 == Feedback and Contributions
 
 For how-to questions or diagnosing or debugging issues, we suggest using StackOverflow,
@@ -122,6 +127,7 @@ information that is not specific to any one version. For example, it has migrati
 notes from earlier versions, comprehensive information on what's new across multiple
 versions, contributor guidelines, the Spring Framework code style, and other information.
 
+[[overview-getting-started]]
 == Getting Started
 
 If you are just getting started with Spring, you may want to begin using the Spring