[DOC] Provide guidance when Migration assistant tool is not a viable option

[pgvillena]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request.

Currently the migration documentation only provides evaluation steps to identify if the tooling is applicable.
After evaluation and the user does not meet prerequisites. It would be good to direct them to what are the alternatives or methods to still proceed with the migration without the use of this tool.
(i.e. if they are currently using ElasticSearch 8.x)

Relavant documentation: https://opensearch.org/docs/latest/migration-assistant/is-migration-assistant-right-for-you/

Version: List the OpenSearch version to which this issue applies, e.g. 2.14, 2.12--2.14, or all.

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.

It would be good to showcase other migration options like the ones listed here in the Prescriptive Guidance.

https://docs.aws.amazon.com/prescriptive-guidance/latest/opensearch-service-migration/build-from-source.html