Add syntax highlighting and Mermaid rendering for org-mode files

Problem

org-mode files in repositories rendered without syntax highlighting or Mermaid diagram support. GitHub has rendered org-mode source blocks with syntax highlighting since around 2010, but this was not supported in GitLab (#32207 (closed)).

What does this MR do?

• Updated CodeLanguageFilter to handle bare <pre lang="python">code</pre> output produced by org-ruby (without a <code> wrapper). This enables syntax highlighting for #+begin_src blocks in org-mode files.
• Added MermaidFilter to MarkupPipeline. This enables Mermaid diagram rendering for #+begin_src mermaid in org-mode files.
• As a side effect, reStructuredText and Textile files also renders Mermaid diagrams (reST and Textile source blocks already had syntax highlighting).
• As a further side effect, code blocks in .rdoc and .creole files gain Mermaid rendering (if the underlying markup parser produces a <pre><code> shape, or <pre lang> that the new wrapping logic normalizes). Based on current testing, .wiki and .mediawiki content does not produce Mermaid‑renderable code blocks with this pipeline.

Screenshots

Manually verified rendering in GDK-in-a-box.

org-mode — syntax highlighting + Mermaid

Before (gitlab.com)	After (GDK-in-a-box)

Source:

#+BEGIN_SRC python
def function():
    s = "Python code"
    print s
#+END_SRC

#+BEGIN_SRC mermaid
graph TD;
    A-->B;
    A-->C;
    B-->D;
    C-->D;
#+END_SRC

reStructuredText — Mermaid

Before (gitlab.com)	After (GDK-in-a-box)

Source:

.. code:: mermaid

   graph TD;
       A-->B;
       A-->C;
       B-->D;
       C-->D;

Textile — Mermaid

Before (gitlab.com)	After (GDK-in-a-box)

Source:

bc[mermaid]. graph TD;
    A-->B;
    A-->C;
    B-->D;
    C-->D;

Notes

• No docs updates included — this area isn't well covered by existing documentation. If there are specific docs that should be updated, please flag them.
• First contribution to GitLab — any guidance on process or conventions would be appreciated.

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