DG: Fix content page formatting

[tllshan]

Let's improve the formatting of the DG by:

• Adding numbering for Appendix D sub-headers
• Adding Appendix D to the contents page
• Fixing broken links in the contents page
• Standardizing placement of line breaks

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (83b6a3f) 89.86% compared to head (22472e5) 89.86%.
Report is 1 commits behind head on master.

❗ Current head 22472e5 differs from pull request most recent head 6b91492. Consider uploading reports for the commit 6b91492 to get more accurate results

Additional details and impacted files

@@            Coverage Diff            @@
##             master     #270   +/-   ##
=========================================
  Coverage     89.86%   89.86%           
  Complexity      797      797           
=========================================
  Files            97       97           
  Lines          2181     2181           
  Branches        294      294           
=========================================
  Hits           1960     1960           
  Misses          162      162           
  Partials         59       59           

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[p-xp]

LGTM!