Bump rexml from 3.2.6 to 3.3.9 in /docs

Bumps [rexml](https://github.com/ruby/rexml) from 3.2.6 to 3.3.9.
- [Release notes](https://github.com/ruby/rexml/releases)
- [Changelog](https://github.com/ruby/rexml/blob/master/NEWS.md)
- [Commits](https://github.com/ruby/rexml/compare/v3.2.6...v3.3.9)

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

Signed-off-by: dependabot[bot] <support@github.com>

docs/Gemfile.lock

@@ -205,11 +205,15 @@ GEM
       rb-fsevent (~> 0.10, >= 0.10.3)
       rb-inotify (~> 0.9, >= 0.9.10)
     mercenary (0.3.6)
+    mini_portile2 (2.8.8)
     minima (2.5.1)
       jekyll (>= 3.5, < 5.0)
       jekyll-feed (~> 0.9)
       jekyll-seo-tag (~> 2.1)
     minitest (5.19.0)
+    nokogiri (1.15.4)
+      mini_portile2 (~> 2.8.2)
+      racc (~> 1.4)
     nokogiri (1.15.4-aarch64-linux)
       racc (~> 1.4)
     octokit (4.25.1)
@@ -222,7 +226,7 @@ GEM
     rb-fsevent (0.11.2)
     rb-inotify (0.10.1)
       ffi (~> 1.0)
-    rexml (3.2.6)
+    rexml (3.3.9)
     rouge (3.26.0)
     ruby2_keywords (0.0.5)
     rubyzip (2.3.2)
@@ -250,6 +254,7 @@ GEM
 
 PLATFORMS
   aarch64-linux
+  x86_64-linux
 
 DEPENDENCIES
   github-pages