polish(v2): url-subpath config validation warning

[slorber]

Motivation

Related to #4925 (comment)

We should make it clearer that user should not use a subpath in site URL config, and rather use the baseurl field:

// docusarus.config.js
module.exports = {
  url: '<domain>/docs' // => BAD: emit a warning
}

In practice, it is an excuse to introduce a validation warning system for the site config, as we'll likely want to enforce stricter config over time, and emitting warning is a good way to progressively deprecate some bad config usage without annoying too much the users

[netlify]

✔️ [V2]

🔨 Explore the source changes: cfd5dbf

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60c8c1063c196c0007ad3c35

😎 Browse the preview: https://deploy-preview-4977--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🔴 Performance	49
🟢 Accessibility	97
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4977--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 623 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/css/styles.********.css	87.6 kB	0 B
website/build/assets/js/main.********.js	444 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	63.4 kB	0 B
website/build/docs/introduction/index.html	235 B	0 B
website/build/index.html	28.2 kB	0 B

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