Migration guide from LMDB to MDBX

[erthink]

Provide a step-by-step guide for migration from LMDB to MDBX.

[pspacek]

As an example https://gitlab.nic.cz/knot/knot-resolver/-/commit/4c8b2e13f8d35f3c807aa0ac6eb976bf62b0493b might be of help, together with rest of https://gitlab.nic.cz/knot/knot-resolver/-/commits/mdbx-experiment branch.