Schemas: add /schemas/list page, and custom 404 for /schemas and missing subpages

[chalin]

@svrnm @tigrannajaryan I think that this is most likely the solution that could meet all of our concerns, as summarized by @svrnm. Here is what this PR offers:

• Fixes /schemas is the top 404 #3468
• Adds a /schemas/list page that lists the published schemas.
• Adds a custom 404 page for /schemas that contains the link to the list page
• Fixes Right nav formatting problem when page has no TOC #1102

Previews

• https://deploy-preview-3508--opentelemetry.netlify.app/schemas - shows custom schemas 404
• https://deploy-preview-3508--opentelemetry.netlify.app/schemas/ - shows custom schemas 404
• https://deploy-preview-3508--opentelemetry.netlify.app/schemas/list/ - shows schemas list page

Redirect tests

• https://deploy-preview-3508--opentelemetry.netlify.app/schemas/latest --> 1.22.0 (as of the time of writing)
• https://deploy-preview-3508--opentelemetry.netlify.app/schemas/latest/ --> 1.22.0 (as of the time of writing)
• https://deploy-preview-3508--opentelemetry.netlify.app/schemas/x.y.z - nonexistent version --> schemas 404

Status tests

$ curl -sI https://deploy-preview-3508--opentelemetry.netlify.app/schemas | grep -E 'HTTP|-type|location' 
HTTP/2 404 
content-type: text/html; charset=UTF-8
$ curl -sI https://deploy-preview-3508--opentelemetry.netlify.app/schemas/ | grep -E 'HTTP|-type|location'
HTTP/2 404 
content-type: text/html; charset=UTF-8
$ curl -sI https://deploy-preview-3508--opentelemetry.netlify.app/schemas/list/ | grep -E 'HTTP|-type|location' 
HTTP/2 200 
content-type: text/html; charset=UTF-8

Screenshots

404 page:

List page:

[svrnm]

lgtm, some minor suggestions

[chalin]

Closing in favor of:

• Add /schemas in YAML, /schemas/list in HTML, and a 404 page for the rest #3537