Players' Manual Updates

[ctm]

Do some of the following to spruce up the Players` Manual.

• Add a link checker to the build process (mdbook-pandoc will do this, but there are dedicated ones, I think)
  • Fix broken links
• Look at the pop-up blocker pull-request (and email) (Pop-Up Enabling in Manual #1320)
• Make an Internals sub-chapter of Esoterica
  • "None of the info should be necessary"
  • "Here's why it might be useful"
  • "This section is even more likely than the rest to be outdated"
    • Move Structures (and its child Templates) to Internals
      • Update Structures
      • Update Templates

My guess is I'll add more items to this as I go, but my goal isn't to make this another rabbit hole (honest!), so I'll try to exercise judgment and add some low-priority issues for later rather than just spinning my wheels indefinitely here.