Document wakefield file format

In OPAL/src!596 the file reading of the wakefield is changed. The manual does not specify what the file format should be, this should be added.

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_Migrated from GitLab_
GitLab ID: 88
Source: https://gitlab.psi.ch/OPAL/documentation/manual/-/issues/88
Original state: opened
Original author: snuverink_j