Feature: support material social cards plugin

[Guts]

This PR adds logic and option to support Social Cards of Material theme out-of-the-box.

Functional

• if a page has no image set but social cards is enabled, use the generated image as item's image
• option to enable/disable this integration in mkdocs.plugins.rss

Before/after

Considering the RSS feed from this plugin documentation: https://guts.github.io/mkdocs-rss-plugin/integrations/

Before

<item>
  <title>Integrations</title>
  <description>&lt;h2&gt;Reference RSS feeds in HTML meta-tags&lt;/h2&gt;
&lt;p&gt;To facilitate the discovery of RSS feeds, it&#39;s recommended to add relevant meta-tags in &lt;code&gt;&amp;lt;head&amp;gt;&lt;/code&gt; section in HTML pa...&lt;/p&gt;</description>
  <link>https://guts.github.io/mkdocs-rss-plugin/integrations/?utm_source=documentation&amp;utm_medium=RSS&amp;utm_campaign=feed-syndication</link>
  <pubDate>Tue, 12 Dec 2023 11:02:08 +0100</pubDate>
  <source url="https://guts.github.io/mkdocs-rss-plugin/feed_rss_created.xml">MkDocs RSS Plugin</source>
  <guid isPermaLink="true">https://guts.github.io/mkdocs-rss-plugin/integrations/</guid>
</item>

After

<item>
    <title>Integrations</title>
    <description>
    <h2>Social Cards plugin (from Material theme)</h2> <p>Since version 1.10, the plugin integrates with the [Social Cards plugin (shipped with Material theme)](https://...</p>
    </description>
    <link>
    http://127.0.0.1:8000/mkdocs-rss-plugin/integrations/?utm_source=documentation&utm_medium=RSS&utm_campaign=feed-syndication
    </link>
    <pubDate>Tue, 12 Dec 2023 12:02:08 +0100</pubDate>
    <source url="http://127.0.0.1:8000/mkdocs-rss-plugin/feed_rss_created.xml">MkDocs RSS Plugin</source>
    <guid isPermaLink="true">
    http://127.0.0.1:8000/mkdocs-rss-plugin/integrations/
    </guid>
    <enclosure url="https://guts.github.io/mkdocs-rss-plugin/assets/images/social/integrations.png" type="image/png" length="46533"/>
</item>

See the last enclosure line.

Resources

• related section about Social Cards in Material documentation: https://squidfunk.github.io/mkdocs-material/setup/setting-up-social-cards/

[codecov]

Codecov Report

Merging #217 (fb2d8d4) into main (27f9d54) will increase coverage by 0.51%.
Report is 10 commits behind head on main.
The diff coverage is 86.36%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #217      +/-   ##
==========================================
+ Coverage   81.92%   82.43%   +0.51%     
==========================================
  Files           9       10       +1     
  Lines         437      518      +81     
  Branches       97      108      +11     
==========================================
+ Hits          358      427      +69     
- Misses         51       58       +7     
- Partials       28       33       +5     

Files	Coverage Δ
mkdocs_rss_plugin/config.py	100.00% <100.00%> (ø)
mkdocs_rss_plugin/plugin.py	94.95% <100.00%> (+0.13%)	⬆️
mkdocs_rss_plugin/util.py	72.64% <83.33%> (+0.25%)	⬆️
...lugin/integrations/theme_material_social_plugin.py	85.93% <85.93%> (ø)