Updating the PRE-IMAGE proof example in Overview page

[noushkia]

The 'EXAMPLE OF HOW TO PROVE KNOWLEDGE OF A PRE-IMAGE' on the Overview page is outdated and doesn't run on the current version of gnark.
The old example
Circuit definition is wrong (for example mimc.NewMiMC(api.Curve()) shouldn't receive a curve as input, the API is sufficient).

[noushkia]

The following section needs to be updated as well:
https://docs.gnark.consensys.net/HowTo/write/standard_library