VIGNETTES: Vignette file names should not start with numbers

[cemacrr]

To avoid package build WARNING, vignette file names should not start with numeric characters:

* checking package subdirectories ... WARNING
Subdirectory 'inst/doc' contains invalid file names:
  ‘01_pfwim_workflow.Rmd’ ‘02_metawebs.Rmd’ ‘03_realised_webs.Rmd’
  ‘01_pfwim_workflow.html’ ‘02_metawebs.html’ ‘03_realised_webs.html’