Update documentation by mentioning %localappdata%\NINA\PlateSolver\Failed

[Han K]

The following issue was solved some time ago:

https://bitbucket.org/Isbeorn/nina/issues/1133/mechanism-to-support-users-with-image

but I could not find it in the online documentation. Suggest to add the following to the documentation:

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The last image which fails to solve (and the .ini file) is stored at the following location for further analysing:

\%localappdata%\NINA\PlateSolver\Failed

---

Thanks, Han

[Stefan B]

Added the location to the plate solver page