Change log level for non-found files during PDF indexing

[koppor]

The PDF indexer writes all non-found files in the log.

Example:

2023-03-17 11:45:05 [pool-1-thread-3] org.jabref.logic.pdf.search.indexing.PdfIndexer.writeToIndex()
WARN: Could not find C:/Users/Oliver/Documents/Arbeiten/DAs/3 - finished/2012 - Splitting BPEL/DIP 3255 - Splitting BPEL Processes - comments OK.pdf

It further outputs the index path:

INFO: Index path for C:\TEMP\cd\test-cd.bib is C:\Users\koppor\AppData\Local\org.jabref\JabRef\lucene94

This is IMHO too much noise. It is required for debugging.

I think, the reason was that log.txt never contained DEBUG lines. This is fixed in this PR.

One can enable debug using tinylog.properties as follows:

level@org.jabref.logic.autosaveandbackup.BackupManager = debug

See https://devdocs.jabref.org/code-howtos/logging.html for details. That howto was not that good, so this PR also refines that howto @calixtus .

• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.