Use absolute internal links in migration guides (Qiskit#1761)

This is to prepare for us moving the migration guides to not have `/api`
in their URL. When we move those files, all the relative links like
`../qiskit` stop working.

The change was generated by find-and-replace, and the link checker will
validate it's all good.

Author: Eric-Arellano

docs/api/migration-guides/index.mdx

@@ -9,18 +9,18 @@ description: Migrate to using the newest from Qiskit and Qiskit Runtime
 These migration guides help you more effectively use Qiskit and Qiskit Runtime:
 
 * Migrate to Qiskit 1.0
-   * [Qiskit 1.0 packaging changes](/api/migration-guides/qiskit-1.0-installation)
-   * [Qiskit 1.0 feature changes](/api/migration-guides/qiskit-1.0-features)
-* [Migrate to the Qiskit Runtime V2 primitives](/api/migration-guides/v2-primitives)
-* [Qiskit Runtime execution mode changes](/api/migration-guides/sessions)
+   * [Qiskit 1.0 packaging changes](./qiskit-1.0-installation)
+   * [Qiskit 1.0 feature changes](./qiskit-1.0-features)
+* [Migrate to the Qiskit Runtime V2 primitives](./v2-primitives)
+* [Qiskit Runtime execution mode changes](./sessions)
 * Migrate to Qiskit Runtime
    * [Overview](./qiskit-runtime)
    * [Migrate from `qiskit-ibmq-provider`](./qiskit-runtime-from-ibmq-provider)
    * [Migrate from `qiskit_ibm_provider`](./qiskit-runtime-from-ibm-provider)
    * [Migrate options](./qiskit-runtime-options)
    * [Common use cases](./qiskit-runtime-use-case)
    * [Examples](./qiskit-runtime-examples)
-* [Migrate from cloud simulators to local simulators](/api/migration-guides/local-simulators)
+* [Migrate from cloud simulators to local simulators](./local-simulators)
 * Qiskit 0.44 changes
    * [`qiskit.algorithms` new interface](./qiskit-algorithms-module)
    * [`qiskit.opflow` deprecation](./qiskit-opflow-module)