docs(v2): Move sidebar content to its own page

[ArtFlag]

Motivation

Related to #3893 and #3894.

TOCs or sidebars are extremely important in any docs website.

In the current version, the sidebar info is inside the intro page and the sidebar sections are from H3 and deeper, meaning they do not appear in the right-hand TOC making it difficult to browse and understand.

With this PR, the sidebar info is in its own page. It appears in the sidebar, and the on-page TOC is useful again.

I've touched up the content a little bit to reduce word count, also see the Test plan section below.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

Please take a solid look at the Understanding sidebar items section.
Task-oriented docs are generally much better for new users, so I've reworked the titles and description to tell us what each type does immediately, instead of telling us the name of each type first (also helps with SEO).

Let me know if this is correct or if needs improvements, I'll take care of it swiflty.

The rest has only changed a little.

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[github-actions]

Size Change: +15 B (0%)

Total Size: 154 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/blog/2017/12/14/introducing-docusaurus/index.html	20.7 kB	-3 B (0%)
website/build/docs/introduction/index.html	180 B	0 B
website/build/index.html	5.82 kB	-1 B
website/build/main.********.js	109 kB	+19 B (0%)
website/build/styles.********.css	17.5 kB	0 B

_{compressed-size-action}

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[slorber]

Thanks, that looks like a good idea to me, but will be easier to review if the deploy preview works :)

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	56
🟢 Accessibility	99
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-3899--docusaurus-2.netlify.app/classic/

[slorber]

@ArtFlag there is now a problem in the deploy preview:

https://deploy-preview-3899--docusaurus-2.netlify.app/classic/docs/sidebar

Also, can you comment in this PR the parts of the doc you modified?

As the content is both moved/updated, highlighting the major doc changes would be helpful, but overall it looks good

[ArtFlag]

Thanks for fixing the issues!

I've mostly modified the section about the type property.

The rest of the page has seen some modifications but mostly to reduce word count.

For example, for the shorthand note that you mentioned: we used to give the shorthand first and then explain what the "normal" version is.
I flipped it around because we talk about the side item object in this page, so didactically, we should probably show the non-shorthand version first... which actually shows the sidebar item object 😉

No content has been deleted.

[slorber]

Thanks

Can you fix the git conflict and the admonition mistake so that I can merge this?
(let me know if you need help)

[ArtFlag]

It should be fine now.

FYI I couldn't push my docs update because pre-commit thought it was an empty commit. Not sure if this update came from my rebase on master, but if there's a way to fix (ignore docs?) that would be nice 👍

[slorber]

thanks

not sure what you are talking about but that looks fine to merge :)