Add Redis compatibility reference to the migration page

[kovan]

Integrate Redis compatibility guidance into the existing migration documentation.

This updates topics/migration.md to cover:

1. Supported migration paths from Redis OSS 2.x through 7.2.x to Valkey 7.2 or later.
2. Redis CE 7.4+ data-file incompatibility.
3. Protocol compatibility through RESP2 and RESP3.
4. RDB/AOF persistence compatibility with Redis OSS 7.2.
5. Redis-style configuration file compatibility.
6. CLI compatibility between redis-cli and valkey-cli.
7. INFO compatibility, including fixed redis_version:7.2.4 and actual Valkey detection through server_name and valkey_version.
8. Lua scripting compatibility for the redis namespace, with Valkey-specific server namespace and version globals.
9. Module API compatibility for existing RedisModule_* modules and new ValkeyModule_* modules.

The migration page now keeps this compatibility context in one place instead of adding a separate reference page or duplicating the existing migration matrix.

Fixes #14

[zuiderkwast]

There is already a page about migration from Redis. We should combine them. We can also consider renaming the existing page. Please spend some time on the full picture here. I don't like just adding more pages without maintaining the old ones.

[kovan]

Updated in d3f7e14. I merged the Redis compatibility material into the existing migration page, removed the separate redis-compatibility page, and updated the topics index so the migration page covers both migration steps and compatibility context.

[zuiderkwast]

Good, but don't just add text without reading the existing text.

The old + new text together create duplicated information. Look at these sections together:

## Why to migrate to Valkey?

* Valkey is the vendor-neutral and open-source software
* Enhanced performance with multi-threading and dual-channel replication
* Improved memory efficiency by using one dictionary per slot in cluster mode and embedding keys in dictionaries. 

### Migration compatibility matrix

You can migrate a Redis server to Valkey.
Valkey is compatible with Redis OSS 7.2 and all earlier open source Redis versions, as Valkey 7.2.4 is a fork of Redis 7.2.4.
Migrating from any open source Redis version to Valkey is effectively an upgrade.

> NOTE: In this guide, whenever a reference to a `redis-cli` or `valkey-cli` command is provided, the reference will only point to the Valkey version of the documentation.

Redis Community Edition (CE), versions 7.4 and later, are not open source and the data files are not compatible with Valkey.
It may be possible to migrate the data to Valkey from proprietary Redis versions and other Redis-like software, but it requires another method and is not covered by this document.

The following table provides migration options depending on the Redis version you run:

| Redis                 | Valkey |
|-----------------------|--------|
| OSS 2.x - 7.2.x       | 7.2.x  |
| OSS 2.x - 7.2.x       | 8.0    |
| CE 7.4                | n/a    |

## Redis compatibility

Valkey is a fork of Redis OSS 7.2.4 and maintains backward compatibility with Redis OSS 7.2 and all earlier open-source Redis versions.
Valkey versions 8.0 and later add new features on top of this base.
These features are Valkey-specific and are not present in Redis OSS.

See?

IMO, this needs manual human work. The LLMs just add more content without taking care of the the existing parts. They are really bad at this.

Als, the old sections are outdated. They announce some of Valkey 8.0 features as the reason to update, but we have many other features now. Better not mention them at all, just that Valkey has many new features and improvements.

[zuiderkwast]

Looks good now. Thank you!

I added some comments, mostly for the old text that was not added in this PR. I can apply the changes before merging.

[zuiderkwast]

Since Redis 8 with AGPL technically is open source (although the license is not permissive) so we can clarify that Valkey is using the original BSD license.

Suggested change

	Valkey is a vendor-neutral, open-source continuation of Redis OSS.
	Valkey is a vendor-neutral, open-source continuation of Redis OSS under the original BSD license.