Refresh docs/getting_started.md.
Refresh docs/getting_started.md.

* Updated paths after several code moves
* Documented a few recently added directories
* Adjusted wording in a few places

Fixes https://github.com/google/iree/issues/435

Closes https://github.com/google/iree/pull/449

COPYBARA_INTEGRATE_REVIEW=https://github.com/google/iree/pull/449 from ScottTodd:docs-getting-started 0dd05aae3f9f3af94362acbf7f14c1f54d067586
PiperOrigin-RevId: 289170810

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