[DOCS] Write a better migration documentation for users upgrading

With the change to a single database in #1575 and the updates to meilisearch versions, users pulling new version of the code, are unable to run the backend without manual help.

We should write a migration documentation that gives explicit steps for users facing such issues.

Error messages:
```
Error: Your database version (1.2.1) is incompatible with your current engine version (1.8.4).
```

```
MeiliSearchCommunicationError: fetch failed
```

```
[Node] Error: getaddrinfo ENOTFOUND graasp-postgres
[Node] at GetAddrInfoReqWrap.onlookupall [as oncomplete] (node:dns:120:26) {
[Node] errno: -3008,
[Node] code: 'ENOTFOUND',
[Node] syscall: 'getaddrinfo',
[Node] hostname: 'graasp-postgres'
[Node] }
```