[MTOOLCHAINS-52] Explain what a toolchain is #110

Elliotte Rusty Harold opened MTOOLCHAINS-52 and commented

The page at https://maven.apache.org/plugins/maven-toolchains-plugin/toolchains/custom.html never explains what a custom toolchain is, or how it differs from a toolchain. It should start with this information.

In fact, nothing I see on the maven-toolchains-plugin website ever says what a toolchain is. I think this might help explain why I've found this so hard to understand. We need to make sure the docs clearly define "toolchain". Either that or don't use the word at all.

Affects: 3.2.0