Add Rails Mermaid ERD gem

Why these changes are being introduced:

EngX has been discussing ways to add more visual documentation
to our applications. The Rails Mermaid ERD gem is one way to
achieve that.

Relevant ticket(s):

N/A

How this addresses that need:

This adds the Rails Mermaid ERD gem. However, it does not include the
actual ERD markdown, because it doesn't work just yet (see side effects).

Side effects of this change:

GitHub cannot currently render rich display Mermaid diagrams. Apparently,
this has been fixed upstream, but GitHub is a few versions behind. Once this
is fixed in GitHub, we should generate the diagram, include it in the readme,
and update the PR template.

.gitignore

@@ -27,3 +27,6 @@ coverage
 
 storage
 .vscode
+
+# Ignore Rails Mermaid ERD gem generated HTML. (We put the markdown in the readme instead.)
+docs/etd_erd.html

Gemfile

@@ -53,6 +53,7 @@ group :development do
   gem 'dotenv-rails'
   gem 'letter_opener'
   gem 'listen'
+  gem 'rails-mermaid_erd'
   gem 'rubocop'
   gem 'rubocop-rails'
   gem 'scout_apm'

Gemfile.lock

@@ -449,6 +449,7 @@ DEPENDENCIES
   pg
   puma
   rails (~> 7.0)
+  rails-mermaid_erd
   rubocop
   rubocop-rails
   rubyzip

config/mermaid_erd.yml

@@ -0,0 +1,3 @@
+# result path
+# default "mermaid_erd/index.html"
+result_path: docs/etd_erd.html