Improvements related to mermaid

• mermaid graphs should ends with .mmd extension following https://mermaid.js.org/ecosystem/integrations-create.html
• quoting labels (error observed with "=" in labels)

assigned to @gmarchment

assigned to @bbrancotte and unassigned @gmarchment

requested review from @bbrancotte

approved this merge request

requested review from @gmarchment and removed review request for @bbrancotte

mentioned in commit 4ba563e8

merged