Add docs for expansion-panel and accordion

[jelbourn]

We currently are missing documentation for expansion-panel / accordion. This needs:

• An overview file (expansion.md)
• At least two examples (typically one simple and one more in-depth)

The style of the overview should align with existing docs.

[julianobrasil]

@jelbourn, there's an issue with expansion-panel. When you select, for the first time, an md-tab that has an expansion inside, the expansion is shown expanded and a little bit messed up: #5269

plunker: http://plnkr.co/edit/q6kXPH?p=preview

If it's put in the examples tab, this issue will mess with it unless some workaround is used or the problem is fixed.

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