Measure the relationship between mirror voltage and beam position

Following https://github.com/DiamondLightSource/i19-bluesky/issues/122 we have a way of getting the beam position. To be able to move the beam to this position we would need to know what voltage change would relate in what beam movements (at least initially a rough idea and maybe some algorithm to iterate it further. We should be able to take this data in an automated way by:

1. Manually move the beam to one edge of the OAV
2. Change the voltage by a small amount
3. Wait for settling
4. Read beam position
5. Save the voltage -> beam position mapping to file

This probably should be repeated a few times in both directions to get some idea of repeatability. We should be able to do this using the `bluesky` `scan` command.

## Acceptance Criteria
- The above data is taken
- The script is available for future use and well documented

Start by reviewing issue 122 and the existing beam-position functionality, then inspect how this repository uses the Bluesky scan command. Use the listed OAV voltage and settling sequence to collect repeated mappings in both directions, save the results to a file, and document the script for future use.

Written by the indexing model from the issue text.