Convert the plugin site pages from xdoc to Markdown

[slachiewicz]

Converts the eight modello-plugin-* index pages from xdoc to Markdown with doxia-converter, leaving download.xml.vm alone.

Please merge or rebase rather than squash — the rename is in its own commit so git log --follow keeps working, and squashing collapses it back into a delete plus an add.

Method signatures become fenced java blocks

In xdoc each signature was a bulleted list item wrapping a <code> span, with <br/> and a run of   standing in for the line break and the indent before throws:

<li><code>public <i>RootClass</i> read( Reader reader, boolean strict )<br/>
  &#160;&#160;&#160;&#160;throws IOException, XmlPullParserException</code></li>

They are now fenced ```java blocks, with consecutive overloads of the same method in one block:

```java
public RootClass read( Reader reader, boolean strict )
    throws IOException, XmlPullParserException

public RootClass read( Reader reader )
    throws IOException, XmlPullParserException
```

The line break and the indent are real characters instead of markup, and the skin renders the block as <pre class="prettyprint"><code class="language-java">, so the syntax highlighter picks it up — which the old <code> spans never got. Class names appearing inside a sentence stay inline code spans.

What this gives up

The <i> and <b> in those spans marked which part of a name the reader substitutes and which part is fixed — <i>my.model.package</i><b>.io.xpp3.</b><i>ModelName</i><b>Xpp3Reader</b>. A Markdown code span cannot carry that, so it is gone. Keeping it would have meant writing every signature as inline HTML in an otherwise plain document; the highlighting is worth more than the emphasis.

Verification

The site was rendered before and after and the two compared page by page on title, author, visible text and link targets. The only differences are the two intended ones — list items became code blocks, and the emphasis inside code spans is gone. Titles, authors and every link target are unchanged.

The YAML front matter is kept and must stay the first bytes of each file: it carries the title and author, and without it the parser falls back to the first heading and <meta name="author"> disappears. Plexus parent 26 already excludes **/src/site/markdown/** from spotless's flexmark formatter, which would otherwise rewrite the fence closing the front matter; spotless:check passes.