ex: adds usage examples for Hacktoberfest

[ChrisKeefe]

This PR is part of our 2022 Hacktoberfest team-wide sprint to add usage examples to all Actions in each of our core distribution plugins. The Actions examples have been added for are as follows:

tabulate

NOTE: tabulate should get an additional example for merging multiple metadata files
Edit: multiple files example finished, will push when wifi allows

[ChrisKeefe]

Tests clean locally, renders in CLI like this:

Examples:
  # ### example: basic tabulate usage
  qiime metadata tabulate \
    --m-input-file demux-stats.qza \
    --o-visualization demux-stats-viz.qzv
  
  # ### example: tabulate multiple files
  # Multiple metadata files or artifacts viewed as metadata can be merged to
  # make one tabular visualization. This one displays only 25 metadata rows
  # per page.
  qiime metadata tabulate \
    --m-input-file sample-metadata.tsv faith-pd-vector.qza \
    --p-page-size 25 \
    --o-visualization demux-stats-viz.qzv