Add section on website modes

[jamesmbaazam]

This PR fixes #100 by adding a subsection under the "documentation" section to layout the rational for using the "unreleased" development mode for pkgdown.

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[joshwlambert]

Thanks for contributing to resolve issue #100 @jamesmbaazam!

I've left a couple of comments within the text.

I would also prefer if this was in it's own blueprints page. I think the information added is too specific to be considered a general principle. Instead I would like if there was a dedicated page for Epiverse-TRACE package websites. For now it can be a small page, just including this information, but gives us a place to expand on things like the website theme, etc.

[joshwlambert]

Thanks for incorporating my previous requests @jamesmbaazam. I've requested a couple of minor changes in the text but overall it's looking good.

[joshwlambert]

A .Rproj file has been added in this PR. Not sure if this should be removed before merging.

[jamesmbaazam]

A .Rproj file has been added in this PR. Not sure if this should be removed before merging.

Best not to introduce it, so I've removed it in d782cf3.