fix(v2): sidebar_label should be used to compute next/previous button labels

[slorber]

Motivation

Fixes #4929

Our doc says:

sidebar_label: The text shown in the document sidebar and in the next/previous button for this document. If this field is not present, the document's sidebar_label will default to its title

This behavior makes sense as the paginated navigation is related to the sidebar, and in both cases we have constrained width and need to use shorter text-only (non-jsx) labels.

In practice, I think this behavior has never been implemented, so let's implement it now + add tests to make sure it stays there

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

tests

[netlify]

✔️ [V2]

🔨 Explore the source changes: e2fa41c

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60c873c290aad900073ee0a3

😎 Browse the preview: https://deploy-preview-4970--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	68
🟢 Accessibility	97
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4970--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 621 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/css/styles.********.css	87.8 kB	0 B
website/build/assets/js/main.********.js	444 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	62.2 kB	0 B
website/build/docs/introduction/index.html	235 B	0 B
website/build/index.html	27 kB	0 B

_{compressed-size-action}