Add translation support for mermaid

Hi, Through trial and error, I was able to figure out, that sphinx-intl can cope with mermaid graphs, if you include them as files and then use the same naming convention as for figure translations. I.e. you include the mermaid-graph as file, then translate it and save it to a new file, following the naming convention. In that case, the rightly translated graph will be picked up.

In the Sphinx options for internationalization, you can set the figure_language_filename to enable internationalization for figures. By default, this is {root}.{language}{ext}.

Let me give you an example:

First, in the base.rst file of the original language, you include your mermaid graph. The graph is stored in a separate file called mermaid.mmd
```
After this paragraph follows a mermaid graph:

.. mermaid:: mermaid.mmd
 :align: center
```
In the mermaid.mmd file, you have your mermaid graph, i.e.:
```
flowchart TD
S(((Beginning)))
T[End]
S--towards-->T
```
In a separate file, you store your translated mermaid graph with the right language specifier. Suppose, we want to translate our mermaid graph to german, we'd save it in a separate file named mermaid.de.mmd like so.
```
flowchart TD
S(((Anfang)))
T[Ende]
S--bis-->T
```
Now, in the file structure, you put them side by side in the same directory, i.e.
```
./
+ base.rst
+ mermaid.mmd
+ mermaid.de.mmd
```

With this structure, sphinx-intl will pick up the rightly translated mermaid graph for internationalization in your documentation.

However, there are some culprits: In particular, if you are using captions, these are not being caught, when running sphinx-build -M gettext. In other words, the following will show show the english caption in all of your translations and I reckon, that this is part of sphinxcontrib-mermaid.

  .. mermaid:: mermaid.mmd
    :align: center
    :caption: What a beautiful mermaid graph!

I hope this helps,

Jacob

mgaitan / sphinxcontrib-mermaid

Add translation support for mermaid #130