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 and a note about updating
the diagram to the PR template. However, it does not include the
actual ERD markdown, because seems that it doesn't actually work
just yet (see side effects).

Side effects of this change:

There's a Mermaid rendering issue on GitHub. Apparently, [this has
been fixed upstream, but GitHub is a few versions behind](mermaid-js/mermaid#4388).

I think it makes sense to add the gem for now, but I'm open to
waiting until everything is working properly.

.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

@@ -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