docs(guides): Improve intro paragraph formatting and expand custom spans description (#17169)

## DESCRIBE YOUR PR

Minor formatting and content improvements across the guides section:

- Split dense opening paragraphs in `issues-errors`, `logs`, `metrics`,
`querying-traces`, and `session-replay` guides into two sentences for
better readability
- Expanded the `custom-spans` guide intro to clarify what custom spans
add (business context, logical groupings, attributes) and note that they
often appear as parents of auto-generated child spans

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## SLA

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- Please give the docs team up to 1 week to review your PR unless you've
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Thanks in advance for your help!

## PRE-MERGE CHECKLIST

*Make sure you've checked the following before merging your changes:*

- [ ] Checked Vercel preview for correctness, including links
- [ ] PR was reviewed and approved by any necessary SMEs (subject matter
experts)
- [ ] PR was reviewed and approved by a member of the [Sentry docs
team](https://github.com/orgs/getsentry/teams/docs)