Restructure concept pages for instrumentation

[svrnm]

This PR aligns the structure of the concept pages for instrumentation with what we have for the language-specific pages (sub pages "manual", "automatic" and "libraries"), this hopefully also allows people to find those pages easier.

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Preview: https://deploy-preview-2763--opentelemetry.netlify.app/docs/concepts/instrumentation/

Redirect tests:

• https://deploy-preview-2763--opentelemetry.netlify.app/docs/concepts/instrumenting/
• https://deploy-preview-2763--opentelemetry.netlify.app/docs/concepts/instrumenting-library/

[chalin]

See inline for a few initial minor comments.

[svrnm]

@chalin any idea why the html checker is failing?

========================================================================
docs/instrumentation/index.html
  target does not exist --- docs/instrumentation/index.html --> /docs/concepts/instrumenting/
========================================================================
✘✘✘ failed in 9.102509125s
1 errors in 393 documents

[chalin]

@svrnm - because our current link checker doesn't understand redirects nor aliases. Therefore, you need to use the canonical path to a page. I just rebased and committed a fix for that path -- see 780f73e.

[svrnm]

@svrnm - because our current link checker doesn't understand redirects nor aliases. Therefore, you need to use the canonical path to a page. I just rebased and committed a fix for that path -- see 780f73e.

Thank you @chalin!

@chalin @cartermp any more feedback or are we good to merge?