Docs for most public functions for formatting DateTimes and Numbers

[ntwilson]

Description of the change This PR just adds documentation for most of the existing public functions in the Data.Formatter.DateTime and Data.Formatter.Number modules. Intended to help with #60, though I don't know if it's fair to say it fixes it. I aired on the verbose side, so let me know if I need to do some pruning.

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Checklist:

• [x] Added the change to the changelog's "Unreleased" section with a link to this PR and your username
• [x] Linked any existing issues or proposals that this pull request should close
• [x] Updated or added relevant documentation in the README and/or documentation directory
• [x] Added a test for the contribution (if applicable)

[garyb]

🤦‍♂️ I thought I'd already returned to this.

Thanks very much for working on it an apologies about the extremely slow turnaround on my part!

[ntwilson]

No worries! Glad to help out.