Fix MDX syntax, broken links, and use of the observability variable

[colleenmcginnis]

Description

Fixes syntax errors, broken links, and updates based on the updated observability variable found while working on elastic/docs#3096.

(This will make it easier to pull in more updates ahead of finishing elastic/docs#3096.)

Documentation sets edited in this PR

Check all that apply.

• Stateful (docs/en/observability/*)
• Serverless (docs/en/serverless/*)
• Integrations Developer Guide (docs/en/integrations/*)
• None of the above

Related issue

N/A

Checklist

• Product/Engineering Review
• Writer Review

Follow-up tasks

Select one.

• This PR does not need to be ported to another doc set because:
  • The concepts in this PR only apply to one doc set (serverless or stateful)
  • The PR contains edits to both doc sets (serverless and stateful)
• This PR needs to be ported to another doc set:
  • Port to stateful docs: <link to PR or tracking issue>
  • Port to serverless docs: <link to PR or tracking issue>

[dedemorton]

LGTM--just had a question.

I've eyeballed this (did not look at the preview).

[elasticdocs]

🚀 Built elastic-dot-co-docs-preview-docs successfully!

• https://elastic-dot-co-docs-production-k9z25nfg7-elastic-dev.vercel.app

• Built with commit 6c9a212

Issues? Visit #next-docs in Slack

[colleenmcginnis]

I'll try to fix these in the script instead.

[colleenmcginnis]

@dedemorton this is ready for a second look! (I pushed a commit so it dismissed your earlier approval.)

[dedemorton]

LGTM