General updates

[pdmosses]

This PR aims to make the template easier to use for those new to Jekyll and the Just the Docs theme. The changes in this PR are merely my suggestions (based on my own lack of understanding of what terms like "deploy" really mean).

• Updates the home page at index.md to give an overview of what the template can do.
• Updates README.md to focus on how to do it (preview).
• Minor polishing of .gitignore, _config.yml, and Gemfile.

When the template has been finalised, I'd like to add a reference to it on the theme home page.

[netlify]

✅ Deploy Preview for just-the-docs-template ready!

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[pdmosses]

@mattxwang I don't see how to request a review for this PR in the usual way…

[mattxwang]

No worries on the requesting review portion; this LGTM. Left one nit but we can address post-merge.

[mattxwang]

99% sure we may need to also eventually add other platforms, but we can do that separately!

[mattxwang]

See: 88e466d

[pdmosses]

@mattxwang thanks for approving and merging this PR! I was expecting that you'd suggest some changes. Anyway, the template is something that we can easily update later, based on user feedback and experience.

When I next get a bit of spare time (hopefully this week) I'll try submitting a PR for the theme docs home page and README, referring to the template for details.

[mattxwang]

When the template has been finalised, I'd like to add a reference to it on the theme home page.

PR welcome for that as well!