center-aligned monster stat block ability score table columns

[mpanighetti]

• center-aligned all ability score columns in monster stat blocks to reflect SRD 5.1 formatting
  • standardized ability score column widths to simplify find-replace workflows in the future (each column is now the maximum required width to accommodate an ability score of 30 (+10))
• converted asterisks in Deck of Many Things to GitHub Markdown footnotes
• updated links to reflect current repo URL in README
• linked directly to release tags in Changelog