Add documentation for EXISTS operator in Search API

[jordanpadams]

Summary

This PR adds comprehensive documentation for the new EXISTS operator feature in the PDS Search API. The EXISTS operator allows users to query products based on field presence or absence in the OpenSearch index.

Changes

• Added new section "Search by Field Existence" to the Search API Cookbook (docs/source/guides/search/cookbook.rst)
• Documented four key use cases:
  1. Basic field existence: Finding products with specific fields (e.g., bounding box coordinates)
  2. Negation: Finding products missing fields (e.g., ongoing investigations without stop dates)
  3. Pattern matching: Using regex patterns to match multiple fields
  4. Combining operators: Using EXISTS with other query operators

Documentation includes

• Clear explanation of when EXISTS returns true (field present, non-null value, multi-valued field handling)
• Practical curl examples following the existing cookbook format
• Note about error handling when regex patterns match zero fields

Related Issues

• Original requirement: As a user, I want to query for documents where a specific search field exists in the document registry-api#406
• Implementation: Add API support for checking if a search field exists in a result (e.g. not null) registry-api#700

Test Plan

• Documentation builds successfully with sphinx-build
• No new errors or warnings introduced
• Examples follow existing cookbook style and formatting
• Generated HTML renders correctly

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[jordanpadams]

Merging into develop since we do not want this to be pushed online yet.