Update sidebar behavior for v2 docs

[tr0njavolta]

This PR attempts to fix empty index page behavior in the sidebar for v2 docs. Will need to make similar adjustments to nested pages in other versions.

[netlify]

✅ Deploy Preview for crossplane ready!

Name	Link
🔨 Latest commit	35fa1a4
🔍 Latest deploy log	https://app.netlify.com/projects/crossplane/deploys/689e283796806f000942809b
😎 Deploy Preview	https://deploy-preview-973--crossplane.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.
Lighthouse	1 paths audited Performance: 95 (🟢 up 2 from production) Accessibility: 98 (🟢 up 6 from production) Best Practices: 92 (no change from production) SEO: 100 (no change from production) PWA: 70 (no change from production) View the detailed breakdown and full score reports

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[jbw976]

Nice, thanks for taking this point of confusion on quickly @tr0njavolta!

Can you describe the behavior change in a bit more detail in the PR description, so we can understand and get aligned on it more easily? 🙇‍♂️

[jbw976]

And the DCO check is failing, can you sign-off your commit? this will be relevant: https://github.com/crossplane/crossplane/tree/main/contributing#certificate-of-origin

[negz]

I much prefer this sidebar behavior. I do notice though that if you browse directly to https://deploy-preview-973--crossplane.netlify.app/v2.0/get-started/ you still get the blank page. That could be an issue for folks with outdated links.

[tr0njavolta]

Thanks for the reminders @jbw976 🙇 and sorry for the rushed PR.

I've updated the following for master, v1.19, v1.20, v2.0, and v2.0preview:
Sidebar behavior - Dropdown does one thing: drops content down

Moved content of
content/cli/_index.md > cli-installation.md
content/learn/_index.md > additional-resources.md

[negz]

I much prefer this sidebar behavior. I do notice though that if you browse directly to https://deploy-preview-973--crossplane.netlify.app/v2.0/get-started/ you still get the blank page. That could be an issue for folks with outdated links.

#974
https://deploy-preview-974--crossplane.netlify.app/master/get-started/

@tr0njavolta @jbw976 WDYT about this approach? I wonder if we should do both - just expand the sidebar when you click it, but also give an autogenerated list of child pages if you manage to browse to e.g. the 'Get Started' page.

I was initially thinking you'd need to explicitly enter the URL for these pages, but currently we link them directly from e.g. https://docs.crossplane.io/v2.0/.

[negz]

We chatted on Slack and decided to go with just #974 instead.