Versioning

[mattyg]

Just wanted to note -- now that we're getting into code examples with links deep into the rust docs, it seems pretty important to setup versioning so users can see docs for either 0.2.x or 0.3.x

As part of this we should:

• replace all links to the rust docs 'latest', to instead link to a specific version
• clearly display the version the docs are applicable to, with a dropdown to switch