[8.14](backport #4616) Make hero text on obs landing page smaller

[mergify]

Description

Changes the font size for hero text so that it's not glaringly large but still stands out a bit from ordinary text.

Note that I didn't make the Logs tab active as mentioned in the issue because I don't think that's the right approach. I think it introduces unexpected behavior that might be confusing to users who expect the first tab in a series to be shown by default.

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

Closes #3422

Checklist

• Writer Review

Follow-up tasks

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• 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>

---

This is an automatic backport of pull request #4616 done by [Mergify](https://mergify.com).

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