docs: documentation for at() and get() methods

[jemgillam]

Description

Needs review!

• detail on implementing at and get added under the header "Access methods"
• added missing detail about the methods a ListStep can accept
• Saw a missing link while reviewing the methods for a Lambda step

Closes https://github.com/graphile/crystal-pre-merge/issues/497

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Latest commit: bc03aec8e65e84d5e978713baa95f0ca3baac6a1

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[jemgillam]

I've reorgainsed the sections on the step-classes page as discussed

I've added a blurb to the https://grafast.org/grafast/step-library/standard-steps/ page as it is linked to directly from the step-classes page, I wrote a small summary taken from the step-library category page