Fixes wrong breadcrumb for 404 page

[harish-sethuraman]

For 404 route we are showing wrong bread crumb (learn react) and showed home as active but showed options of learn. So removed breadcrumb for 404 route.

It seems like the when the layout was not specified we were taking Learn as the default subtree
https://github.com/reactjs/reactjs.org/blob/7f995398f4cfffe9132341d4380b716d65d3a214/beta/plugins/md-layout-loader.js#L18
So changed the default sub tree to Home and then had to add layout to all the pages inside the learn directory.

Verified changes with both sidenav and mobile nav for both 404 page and valid page.

Before:

After:

Also the navigation highlights Home but shows options of learn.

[github-actions]

Size Changes

📦 Next.js Bundle Analysis

This analysis was generated by the next.js bundle analysis action 🤖

⚠️ Global Bundle Size Increased

Page	Size (compressed)
global	91.19 KB (🟡 +16 B)

Details

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One Hundred Eighty-five Pages Changed Size

The following pages changed size from the code in this PR compared to its base branch:

Page	Size (compressed)	First Load
/	203.17 KB (🟡 +16 B)	294.35 KB
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[lex111]

@gaearon what if we automatically set a layout based on a content file path? Then we don't have to manually specify layout field in frontmatter of MD file.

[gaearon]

Oh yeah that would be great.