Add BI tool migration guide - #1112

Adds a new page walking through migrating from another BI tool (Looker, Tableau, Power BI, Metabase, Mode, etc.) into Lightdash.

The guide covers the process end to end as a numbered walkthrough:

1. Install the Lightdash CLI and skills so an AI coding agent can do most of the heavy lifting.
2. Set up a migration workspace (branch, preview project, migration/source/ folder).
3. Extract source information — code (LookML, dbt), files (.twb, .pbix), API extraction, and warehouse query history for direct SQL.
4. Capture dashboard screenshots so the agent can reproduce layouts.
5. Translate modeling into Lightdash YAML / dbt in passes (models → metrics → joins), with a mapping table for common source metric types.
6. Rebuild dashboards and validate that numbers match the source.
7. Layer in Lightdash best practices and any customer-specific modeling / style conventions, then plan the cutover.

Also adds the page to navigation under Setting up a new project.

Requested by Ian in Slack.

Generated by Mintlify Agent.

Requested by: ian@lightdash.com via Slack
Mintlify session: slack_1786626639.030269_C0AM8D5UUR5