[Bugfix:Developer] Add ERB to Gemfile (#753)

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### Why is this Change Important & Necessary?
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Resolves Submitty/Submitty issue
[#12802](Submitty/Submitty#12802).

### What is the New Behavior?
<!-- Include before & after screenshots/videos if the user interface has
changed. -->
Before: Server fails to start

<img width="1408" height="507" alt="Screenshot_20260418_164524"
src="https://github.com/user-attachments/assets/2a969820-2045-422f-87b2-d5d2d4fe8266"
/>


After: Server starts successfully

<img width="729" height="174" alt="Screenshot_20260418_164537"
src="https://github.com/user-attachments/assets/007c2d3e-7e4e-405a-be20-612a4cef732b"
/>


### What steps should a reviewer take to reproduce or test the bug or
new feature?

1. Try to build the documentation based on instructions in README.md.
2. When running `bundle exec jekyll serve`, observe error
3. Switch to this branch
4. When running `bundle exec jekyll serve`, observe successful build


### Automated Testing & Documentation
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N/A
### Other information
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N/A

Author: Eli-J-Schwartz

Gemfile

@@ -12,3 +12,5 @@ gem "webrick", "~> 1.9"
 gem 'html-proofer', "~> 3.19.4"
 
 gem 'json'
+
+gem 'erb'

Gemfile.lock

@@ -30,6 +30,7 @@ GEM
     em-websocket (0.5.3)
       eventmachine (>= 0.12.9)
       http_parser.rb (~> 0)
+    erb (6.0.3)
     ethon (0.16.0)
       ffi (>= 1.15.0)
     eventmachine (1.2.7)
@@ -295,6 +296,7 @@ PLATFORMS
   x86_64-linux
 
 DEPENDENCIES
+  erb
   github-pages (= 232)
   html-proofer (~> 3.19.4)
   json