docs(investigate): add Explore logs (Logs Drilldown) page

[ps48]

Description

Adds a dedicated Explore logs documentation page under Investigate → Discover and cross-links to it from Discover Logs.

Explore logs (also called the Logs Drilldown) is an onboarding canvas for the Logs experience. It lets a user browse the log data a cluster holds — with a live severity histogram and a preview of the latest raw log lines for each index and dataset — before writing a query, and turn one or more raw indexes into a queryable dataset in a few clicks. This addresses the Logs cold-start problem: "what logs do I have, are they flowing right now, and how do I start?"

The new page covers:

• Prerequisites — the explore.enabled and explore.logsDrilldown.enabled flags (both off by default), an observability workspace, a registered data source under Multiple Data Sources, and the PPL dependency for previews.
• Opening Explore logs — reached from the Explore logs action in the Logs query bar (and the Logs empty state).
• The canvas and Reading a card — the toolbar, the Datasets/Indexes card stack, the severity histogram, log-line preview, time-field control, and index-health pill, plus a table of card states (Full / No recent data / No documents yet / No time field / Couldn't load preview).
• Selecting the time range, Creating a dataset from indexes (the smart wildcard proposal + the Create dataset wizard), and Querying an existing dataset.
• Data requirements — the time-field and severity fields the feature detects.

The Discover Logs page gets a short :::tip + canvas image pointing users who don't yet have a dataset to Explore logs.

Screenshots were captured from a local OpenSearch Dashboards build with the feature enabled and live data. The Starlight link validator passes (npm run build).

Issues Resolved

Relates to opensearch-project/OpenSearch-Dashboards#12395 (RFC)
Relates to opensearch-project/OpenSearch-Dashboards#12407 (implementation)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.62%. Comparing base (fa26d87) to head (380adc1).

Additional details and impacted files

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  Lines         169      169           
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  Hits           94       94           
  Misses         74       74           
  Partials        1        1           

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