feat(v2): add icon to generic sidebar link

[lex111]

Motivation

I've noticed that many docs sites have external links in the sidebar that are visually different from the usual ones.
Typically an icon serves as a visual marker for external link, so I added one to the corresponding sidebar item type.

Have you read the Contributing Guidelines on pull requests?

Yes

Test Plan

{
  type: 'link',
  label: 'External link',
  href: 'https://example.com',
},

Related PRs

(If this PR adds or changes functionality, please take some time to update the docs at https://github.com/facebook/docusaurus, and link to your PR here.)

[github-actions]

Size Change: +489 B (0%)

Total Size: 544 kB

Filename	Size	Change
website/build/assets/css/styles.********.css	87 kB	+489 B (+1%)

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/js/main.********.js	370 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	60.5 kB	0 B
website/build/docs/introduction/index.html	235 B	0 B
website/build/index.html	25.6 kB	0 B

_{compressed-size-action}

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[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟢 Performance	90
🟢 Accessibility	96
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

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[slorber]

why not :)

Not sure everybody will like that but it is easy to disable so let's merge

[samj]

@slorber : not easy enough apparently

[slorber]

@samj every single Docusaurus component can be ejected and overridden. You can swizzle --eject and customize that code, removing the icon if you want to.

Swizzle:

• DocSidebarItemLink for doc sidebar links
• NavbarNavLink for navbar links