Docs: adding "Connecting to databases tutorials"

[agnes512]

Continuation of #6061
Finish the content in tutorials/connectors/Connecting-to-databases.md

Acceptance Criteria

• Add contents:
• Briefly introduce what is connectors and what connectors LB provides.
• List the connector tutorials we have in connector ref (MySQL, PostgreSQL, Oracle, and MongoDB)
• Add the link to connector ref

[stale]

This issue has been marked stale because it has not seen activity within six months. If you believe this to be in error, please contact one of the code owners, listed in the CODEOWNERS file at the top-level of this repository. This issue will be closed within 30 days of being stale.

[stale]

This issue has been closed due to continued inactivity. Thank you for your understanding. If you believe this to be in error, please contact one of the code owners, listed in the CODEOWNERS file at the top-level of this repository.

[mbryk]

This issue is still linked from the "Connecting to Back-end Service Tutorials" page. Will that page be receiving any documentation? Thank you!