Add docs infra

[benjeffery]

Fixes #209

After thinking about it jupyter-book made the most sense as we use it on other projects and it is there if the docs grow.
I think pages in the site could link here rather than trying to fit docs in the app itself.

Any image URL in the markdown that looks like tsbrowse:file.tsbrowse:page_name gets rendered automatically. In this way screenshots in the docs are kept up to date. Filed #214 for completion of content.

[github-actions]

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[coveralls]

Pull Request Test Coverage Report for Build 11484917745

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 90.288%

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Totals
Change from base Build 11478549132:	0.0%
Covered Lines:	753
Relevant Lines:	834

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💛 - Coveralls

[jeromekelleher]

Snazzy!