Bump rexml from 3.2.5 to 3.3.6 in /docs

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.5 to 3.3.6.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](ruby/rexml@v3.2.5...v3.3.6)

---
updated-dependencies:
- dependency-name: rexml
  dependency-type: indirect
...

Signed-off-by: dependabot[bot] <[email protected]>

docs/Gemfile.lock

@@ -1,72 +1,26 @@
 GEM
   remote: https://rubygems.org/
   specs:
-    addressable (2.8.0)
-      public_suffix (>= 2.0.2, < 5.0)
-    colorator (1.1.0)
-    concurrent-ruby (1.1.9)
-    em-websocket (0.5.2)
-      eventmachine (>= 0.12.9)
-      http_parser.rb (~> 0.6.0)
-    eventmachine (1.2.7)
     ffi (1.15.4)
-    forwardable-extended (2.6.0)
-    http_parser.rb (0.6.0)
-    i18n (0.9.5)
-      concurrent-ruby (~> 1.0)
-    jekyll (3.9.1)
-      addressable (~> 2.4)
-      colorator (~> 1.0)
-      em-websocket (~> 0.5)
-      i18n (~> 0.7)
-      jekyll-sass-converter (~> 1.0)
-      jekyll-watch (~> 2.0)
-      kramdown (>= 1.17, < 3)
-      liquid (~> 4.0)
-      mercenary (~> 0.3.3)
-      pathutil (~> 0.9)
-      rouge (>= 1.7, < 4)
-      safe_yaml (~> 1.0)
-    jekyll-sass-converter (1.5.2)
-      sass (~> 3.4)
-    jekyll-seo-tag (2.6.1)
-      jekyll (>= 3.3, < 5.0)
-    jekyll-watch (2.2.1)
-      listen (~> 3.0)
-    just-the-docs (0.3.3)
-      jekyll (>= 3.8.5)
-      jekyll-seo-tag (~> 2.0)
-      rake (>= 12.3.1, < 13.1.0)
     kramdown (2.3.1)
       rexml
     kramdown-parser-gfm (1.1.0)
       kramdown (~> 2.0)
-    liquid (4.0.3)
     listen (3.2.1)
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
-    mercenary (0.3.6)
-    pathutil (0.16.2)
-      forwardable-extended (~> 2.6)
-    public_suffix (4.0.6)
-    rake (13.0.6)
     rb-fsevent (0.11.0)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rexml (3.2.5)
-    rouge (3.26.1)
-    safe_yaml (1.0.5)
-    sass (3.7.4)
-      sass-listen (~> 4.0.0)
-    sass-listen (4.0.0)
-      rb-fsevent (~> 0.9, >= 0.9.4)
-      rb-inotify (~> 0.9, >= 0.9.7)
+    rexml (3.3.6)
+      strscan
+    strscan (3.1.0)
 
 PLATFORMS
   universal-darwin-18
+  x86_64-linux
 
 DEPENDENCIES
-  just-the-docs
   kramdown-parser-gfm
   listen (~> 3.2.1)