Docs: Updated Athena data source docs

[lwandz13]

Summary
Restructures the Amazon Athena data source documentation from a single monolithic _index.md into a multi-page doc set following Grafana's data source documentation standards.

Changes
New structure (docs/sources/):

• _index.md -- Overview, supported features, pre-built dashboards, plugin updates
• configure.md -- Configuration settings, AWS authentication, IAM policies, provisioning examples
• query-editor.md -- Query editor fields, format options, macros, async queries, query result reuse
• template-variables.md -- Variable types, query examples, usage in queries
• annotations.md -- Annotation columns, example queries
• alerting.md -- Alert rule setup, query requirements, performance considerations
• troubleshooting.md -- Authentication, connection, query, and performance issues (new content based on codebase error analysis)

Style and accuracy fixes applied across all pages:

• Added YAML frontmatter (description, keywords, labels, menuTitle, title, weight, last_reviewed, aliases)
• Verified supported features against plugin.json (corrected Logs from Yes to No, added Traces: No)
• Documented previously undocumented features: Logs format option, resource selectors with template variable support, Max Age in Minutes for query result reuse, context-aware autocompletion
• Documented pre-built dashboards (Cost Usage Report Monitoring, Amazon VPC Flow Logs)

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[lwandz13]

@kevinwcyu I made the requested changes - can you re-review? Thank you!