Improve migration documentation with guides for Datasources migration

[chanikag]

Description

We identified that when we do the migrations from EI versions to MI versions, we want to migrate the Datasources [1] (which are created in the EI console) manually to CARApps [2].

Although this is a mandatory guide in the EI to MI migration, we can not find it in the migration documentations (for example : [3]).
Please, can you consider this and add the required guides to the migration documentation?

[1] - https://wso2docs.atlassian.net/wiki/spaces/ADMIN44x/pages/6684809/Configuring+an+RDBMS+Datasource
[2] - https://apim.docs.wso2.com/en/latest/integrate/develop/creating-artifacts/data-services/creating-datasources/
[3] - https://github.com/wso2-enterprise/migration-docs/blob/main/api-manager/migration-docs/apim/apim-4.2.0/mi/upgrading-from-ei-6.6.x.md

Describe the problem(s)

A detailed description of the purpose of this improvement.

Describe your solution(s)

A detailed description of how this improvement will solve the problem described above.

Related issue(s) (optional)

Any related issues such as sub tasks and issues reported in other repositories (e.g., component repositories), similar problems, etc.

Suggested label(s) (optional)

Optional comma-separated list of suggested labels. Non committers can’t assign labels to issues, and thereby, this will help issue creators who are not a committer to suggest possible labels.

Suggested assignee(s) (optional)

Optional comma-separated list of suggested team members who should attend the issue. Non committers can’t assign issues to assignees, and thereby, this will help issue creators who are not a committer to suggest possible assignees.