Use the modified dates of files in the sitemap (#3774)

* The extracted dates are also added as a comment to the `head` of HTML pages

Signed-off-by: Miki <miki@amazon.com>

Gemfile

@@ -22,6 +22,7 @@ gem "jekyll-redirect-from", "~> 0.16"
 
 # If you have any plugins, put them here!
 group :jekyll_plugins do
+  gem "jekyll-last-modified-at"
   gem "jekyll-sitemap"
 end
 
@@ -34,6 +35,7 @@ gem "wdm", "~> 0.1.0" if Gem.win_platform?
 # Installs webrick dependency for building locally
 gem "webrick", "~> 1.7"
 
+
 # Link checker
 gem "typhoeus"
 gem "ruby-link-checker"

_config.yml

@@ -202,10 +202,15 @@ anchor_links: true
 footer_content:
 
 plugins:
+  - jekyll-last-modified-at
   - jekyll-remote-theme
   - jekyll-redirect-from
   - jekyll-sitemap
 
+# This format has to conform to RFC822
+last-modified-at:
+  date-format: '%a, %d %b %Y %H:%M:%S %z'
+
 # Exclude from processing.
 # The following items will not be processed, by default. Create a custom list
 # to override the default setting.

_includes/head_custom.html

@@ -12,3 +12,5 @@
 {% else %}
   <script src="{{ '/docs/latest/assets/js/version-selector.js' }}"></script>
 {% endif %}
+
+<!-- Last-Modified: {% last_modified_at %} -->