Streamline enhancement template

This updates our enhancement template to focus the questions around three general topics:

What (description of the enhancement, at the top)
Why (benefit/value of the enhancement from the user's perspective)
How (implementation details and ideas, either from the issue opener or in a way that we can fill in later)

It also adds a section for tasks to complete so that we can track next-steps if necessary.

executablebooks / .github