Optional Panel Header

[neilsmind]

What does it do?

• Removes panel heading if the panel is not nested in an accordion and has neither header property nor slot
• Adds a Panel documentation page

Why?

Currently, there is no way to create a Panel without a header. This is a common use case that has
and example in the Bootstrap documentation itself.

What else should I know?

This only applies to v2 branch

Affected Issues

• While this could resolve Panels to just have a collapse option? #21, it only applies to v2

cc: @wffranco @yuche @shaoxiong789