Self contained signup example

[DylanSp]

Adds demonstration code for the sign-up form example as a single HTML file; it imports Bulma and jQuery via <link> and <script> tags, so the HTML works on its own without needing any additional setup. I grabbed the Bulma import from their starter template, the jQuery import comes from the minified version of jQuery Core 3.7.1 on the jQuery releases page.

This isn't ready to be merged yet, there's a few things that need to be done: This is now ready to be merged, as of https://github.com/ziolko/spreadapi/pull/14/commits/6dbe7ada86a9562313739823a3713f453c2a7fd2.

• Integrating this with the written documentation in website/examples/sign-up-form.md. Completed.
• The HTML file could probably use some additional explanatory comments. I looked it over, I think it's good to know as it is.
• Deciding if this is the right place in the repo's folder structure to put it. It might be better to put the added HTML file in a separate examples/ folder at the repo's top level, instead of inside the website/ folder that contains everything for Gitbook; I'm not sure.
• I added the .gitignore and package-lock.json files because I was working on this in a Codespace, which automatically runs npm install and npm run build when starting up for the first time. @ziolko, up to you if you want to keep them in or not.

[DylanSp]

I've updated the documentation in website/examples/sign-up-form.md to use the self-contained HTML file I created; I also polished the prose a bit while I was at it. This should be ready to merge now. The only other thing I can think of is if you want to delete the .html file in website/examples and just have the code in the Markdown file; up to you, @ziolko.

[ziolko]

Hi @DylanSp, thank you for update! If you don't mind, please give me a couple of days to review this change :)

[DylanSp]

Not a problem, take as much time as you need.

[ziolko]

Thanks @DylanSp for this example! I've merged it to master.