docs: add PPL Query Builder page

[joshuali925]

Description

This PR adds PPL query builder docs. It adds a new docs page for query builder, and links it in logs discover docs page

Issues Resolved

List any issues this PR will resolve, e.g. Closes [...].

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 55.62%. Comparing base (c6d5505) to head (1fd4080).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #383   +/-   ##
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  Coverage   55.62%   55.62%           
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  Files           4        4           
  Lines         169      169           
  Branches       47       47           
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  Hits           94       94           
  Misses         74       74           
  Partials        1        1           

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[joshuali925]

@joshuali925-osdbot remove ## Enabling the builder section and relevant content added by this PR, and enable those feature flags by default for the observability stack setup, then send a PR

[joshuali925-osdbot]

On it — I'll remove the ## Enabling the builder section from the query builder docs and enable the relevant feature flags by default in the observability stack setup, then open a PR.

[joshuali925-osdbot]

Done — opened #387.

• Enabled explore.logsQueryBuilder.enabled: true by default in the docker-compose OSD config and both Helm values files (values.yaml, values-anonymous-auth.yaml), which already set explore.enabled: true.
• Removed the ## Enabling the builder section and the availability note from ppl/query-builder.md, plus the "how to enable it" reference on the discover-logs page.

Verified the docs build introduces no new broken links (same 41 pre-existing link-validator warnings on both main and the branch) and validated the YAML.

[joshuali925]

@joshuali925-osdbot Send a PR to merge the query builder docs page into discover logs

[joshuali925-osdbot]

On it — I'll open a PR that merges the PPL Query Builder docs page into the Discover Logs page and removes the standalone page.

[joshuali925-osdbot]

Done — opened #390.

• Merged the standalone PPL Query Builder page into the Discover Logs page as a new Building queries with the PPL Query Builder section (keeping all subsections: modes, searching, filtering, aggregating, sorting, running, worked example).
• Removed ppl/query-builder.md and its sidebar entry, and repointed the in-page reference to the new section anchor.

Verified via the docs build that this introduces no new broken links (37 on the branch vs. 41 on main) and previewed locally — Discover Logs renders the merged section and the old page now 404s. Screenshot is in the PR description.