Support for static pre-rendering

[astrojuanlu]

I have been trying all different rendering modes of sphinxcontrib-mermaid, and now I'm stuck:

• I cannot use raw, because of Output shows html code after a search #22
• I cannot use svg, because apparently different browsers will render the image differently Render Mermaid diagrams from Sphinx kedro-org/kedro#2392 (review)
• Lately I cannot use png either, because of intermittent puppeteer issues Timed out after 30000 ms while waiting for the WS endpoint URL to appear in stdout! puppeteer/puppeteer#10556

At this point I wanted to try to pre-render the PNG images while keeping the Mermaid code in the docs (similar to what happens with sphinx.ext.autosummary). But the extension will dump the image directly in the build/html directory:

sphinxcontrib-mermaid/sphinxcontrib/mermaid.py

Line 188 in 6f8de39

outdir = os.path.join(self.builder.outdir, self.builder.imagedir)

Hence I cannot commit the PNG to version control and use it.