PS: Updating and improving placeholder table

[RedstoneFuture]

Overview

• Removed not implemented placeholders
  • %plotsquared_currentplot_world_name% removed
  • %plotsquared_currentplot_localflag_<flag>% removed
  • %plotsquared_currentplot_flag_<flag>% removed
• Adding already added placeholders and added short world notations
• Improving description, small type
• Reformatting Markdown table

Description

I removed 3 placeholders, as I don't found the implementation in the code (PlaceholderRegistry.java, PAPIPlaceholders.java). The old class Placeholders.java with these placeholders was removed.

I added new placeholders and short world notation from the PlotSquared PR 4741 and 4819.

Submitter Checklist

• Make sure you are opening from a topic branch (/feature/fix/docs/ branch (right side)) and not your main branch.
• Ensure that the pull request title represents the desired changelog entry.
• New public fields and methods are annotated with @since TODO.
• I read and followed the contribution guidelines.