[DOC] Migration assistant documentation

[Naarcha-AWS]

What do you want to do?

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

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

@sumobrian and team have built a cloud-agnostic migration tool. Currently, the tool has a AWS-specific integration guide and user-specific wiki documentation. The team would like to transfer over the wiki from the migration assistant repo into the documentation website as its own section, similar to Benchmark or Data Prepper.

*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.

[Naarcha-AWS]

Here are the action items needed to complete this:

• [ ] Move over the migration assistant wiki content to a _migration directory. Wiki content is here (https://github.com/opensearch-project/opensearch-migrations/wiki).
• [ ] Work with @sumobrian to update the content to make migration assistant more cloud agnostic.
• [ ] Add a tile on the OpenSearch home page for migrations.