infra: remove migrating-from-dfx page, link to canonical CLI docs (#140)

## Summary

- Moves the `icp-cli` full documentation link to the first prose mention
of icp-cli in `overview.md` (instead of a standalone line after
installation)
- Deletes `docs/guides/tools/migrating-from-dfx.md` entirely — this file
was synced from `dfinity/icp-cli` and was prone to drift; the migration
guide now links directly to the canonical source at
`https://cli.internetcomputer.org/0.2/migration/from-dfx`
- Updates the migration note placement: moved above "Telemetry opt-out"
at the end of the icp-cli intro

## Sync recommendation

`docs/guides/tools/overview.md` — `informed by dfinity/icp-cli`
`docs/guides/tools/migrating-from-dfx.md` — deleted; canonical content
lives at `https://cli.internetcomputer.org/0.2/migration/from-dfx`

Author: marc0olo