[DOC] Add documentation page for the OpenSearch plugin

[Oliver-ke]

Description

Adds the missing docs/opensearch/ page set, following the layout used by
the other datasource plugins:

• README.md - landing page covering OpenSearchDatasource and
  OpenSearchLogQuery, plus index-pattern handling, automatic time
  filtering, and field mapping for schema-flexible indices
• model.md - YAML spec reference for both kinds and the datasource selector
• go-sdk/datasource.md, go-sdk/log-query.md - Dashboard-as-Code reference

The tags: [datasource] frontmatter also adds OpenSearch to the
"Supported backends" list on the Datasource concept page, so no change is
needed in perses/perses.

Nav registration is a companion PR against perses/website. (perses/website#153)

Screenshots

Checklist

• Pull request has a descriptive title and context useful to a reviewer.
• Pull request title follows the [<catalog_entry>] <commit message> naming convention using one of the
  following catalog_entry values: FEATURE, ENHANCEMENT, BUGFIX, BREAKINGCHANGE, DOC,IGNORE.
• All commits have DCO signoffs.

UI Changes

• Changes that impact the UI include screenshots and/or screencasts of the relevant changes.
• Code follows the UI guidelines.