[DOCS] Augment self-managed connector tutorials

[lcawl]

This PR:

• updates https://www.elastic.co/docs/reference/search-connectors/es-postgresql-connector-client-tutorial to include more serverless details. The updates are in part based on https://www.elastic.co/getting-started/enterprise-search/search-across-business-systems-and-software.
• changes the style a bit (e.g. use of stepper component) to align with work on other getting started content.
• fixes broken links, headings, and style in https://www.elastic.co/docs/reference/search-connectors/api-tutorial
• fixes some out-dated UI navigation in https://www.elastic.co/docs/reference/search-connectors/connectors-ui-in-kibana

[github-actions]

🔍 Preview links for changed docs

• docs/reference/search-connectors/api-tutorial.md
• docs/reference/search-connectors/connectors-ui-in-kibana.md
• docs/reference/search-connectors/es-postgresql-connector-client-tutorial.md
• docs/reference/search-connectors/self-managed-connectors.md

[elasticsearchmachine]

Pinging @elastic/es-docs (Team:Docs)

[leemthompo]

LGTM