Simplify home page to enhance usability

[semioticrobotic]

This pull request suggests several changes aimed at making the Apache Incubator home page at https://incubator.apache.org/ easier to use. It recommends:

• Consolidating material on the project's history, removing duplicative details from /policy/incubation.html and relocating them to the home page (where similar details were already present)
• Outlining initial steps prospective podlings should take to get started with Incubator, foregrounding key documents they should review
• Removing broken hyperlinks
• Re-writing page copy in certain places, to aid readability

[semioticrobotic]

FYI @ShaneCurcuru this is an attempt to being addressing some of your findings.

[semioticrobotic]

@dave2wave: Anything else I'm able to provide here?

[dave2wave]

I was hoping that someone else would review this, but ... I'l merge it now.