fix(v2): navbar doc item fallback: search doc in lastVersion

[slorber]

Motivation

When using a navbar doc item like:

        {
          type: 'doc',
          docId: 'myDocId',
        },

It was previously required that myDocId exist in ALL docs versions of the sites, so that we can always link to the currently active version.

This PR makes it more fail-safe, and falls-back to looking for the doc in lastVersion, so that older versions are not forced to contain a doc with id: myDocId (ie we can more easily introduce a navbar link to an existing versioned site)

Fix #4972

Have you read the Contributing Guidelines on pull requests?

yes

Test Plan

local :'( we don't have much test infra on the theme code and no dogfooding possible

[netlify]

✔️ [V2]

🔨 Explore the source changes: 653cc5d

🔍 Inspect the deploy log: https://app.netlify.com/sites/docusaurus-2/deploys/60c9d8c614c06a0008636036

😎 Browse the preview: https://deploy-preview-4985--docusaurus-2.netlify.app

[github-actions]

⚡️ Lighthouse report for the changes in this PR:

Category	Score
🟠 Performance	56
🟢 Accessibility	97
🟢 Best practices	100
🟢 SEO	100
🟢 PWA	95

Lighthouse ran on https://deploy-preview-4985--docusaurus-2.netlify.app/

[github-actions]

Size Change: 0 B

Total Size: 624 kB

ℹ️ View Unchanged

Filename	Size	Change
website/build/assets/css/styles.********.css	87.6 kB	0 B
website/build/assets/js/main.********.js	445 kB	0 B
website/build/blog/2017/12/14/introducing-docusaurus/index.html	63.3 kB	0 B
website/build/docs/introduction/index.html	235 B	0 B
website/build/index.html	28.1 kB	0 B

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