Add Rails Mermaid ERD gem and generated ERD diagram

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 and includes the generated
ERD markdown to our readme. It also adds a note about updating
the diagram to the PR template.

Side effects of this change:

None.

.github/PULL_REQUEST_TEMPLATE.md

@@ -6,14 +6,15 @@
       [our guide](https://mitlibraries.github.io/guides/basics/a11y.html) and
       all issues introduced by these changes have been resolved or opened as new
       issues (link to those issues in the Pull Request details above)
+- [ ] ERD has been updated (if applicable)
 - [ ] Stakeholder approval has been confirmed (or is not needed)
 
 #### Code Reviewer
 
 - [ ] The commit message is clear and follows our guidelines
       (not just this pull request message)
 - [ ] There are appropriate tests covering any new functionality
-- [ ] The documentation has been updated or is unnecessary
+- [ ] The documentation, including ERD, has been updated or is unnecessary
 - [ ] The changes have been verified
 - [ ] New dependencies are appropriate or there were no changes
 

.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

@@ -295,6 +295,8 @@ GEM
     rails-html-sanitizer (1.6.0)
       loofah (~> 2.21)
       nokogiri (~> 1.14)
+    rails-mermaid_erd (0.4.2)
+      rails (>= 5.2)
     railties (7.0.5.1)
       actionpack (= 7.0.5.1)
       activesupport (= 7.0.5.1)
@@ -444,6 +446,7 @@ DEPENDENCIES
   pg
   puma
   rails (~> 7.0)
+  rails-mermaid_erd
   rubocop
   rubocop-rails
   rubyzip