migrations: clearly state that the LMS needs data movement tooling

[ajstorm]

Adam Storm (ajstorm) commented:

The LMS is meant to proxy data to multiple database in the end-stage of a migration, once all of the data has been seeded on the target database and continuous replication is in place. We do call this out in various places through the LMS docs, but the docs are long, and users don't always read them from top to bottom.

It would be beneficial to add a short section at the top of the docs which describes the prerequisites for using the LMS, namely that the data must be fully moved from source to target (using some other tooling - ideally Fetch, when it's GAed), and some replication mechanism must be setup to continually replicate data (at least for shadowMode = none).

Jira Issue: DOC-9598

[exalate-issue-sync]

Ryan Kuo (taroface) commented:
Context: https://cockroachlabs.slack.com/archives/C02TLESRWCB/p1706281467155219