Initialise package

[rmbielby]

Brief overview of changes

This PR sets the eesyapi repo up as a package with the base documentation (largely following dfeR and dfeshiny) and a first function: api_url(), which is used to render the url for a given API endpoint.

Why are these changes being made?

API users working in R need a set of helper functions to smooth access to the EES API and this lays the groundwork for us to create those functions.

Detailed description of changes

Setting up the package and adding a first function:

• Created a basic readme for the repo
• Copied across standard contributor guidelines, code of conduct etc from dfeshiny
• Initiated pkgdown to render documentation to github.io
• api_url() renders a url for an endpoint based on a set of parameters, including endpoint and dataset_id
• created a basic set of tests for api_url()
• set up workflows following what we have on dfeshiny

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[cjrace]

Left a few comments, all very minor / quick ones. Couple of lintr things flagged too.

Also noticed this note - assuming it's because httr is in there ready for when we do use it and it's just giving a note because we're not using it yet, so you can ignore it.