#1542 Making sure config and directives works for overriding theme variables using initialize call or directives + tests

docs/developer-docs/configuration.md

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+# Configuration
+
+When mermaid starts configuration is extracted to a configuration to be used for a diagram. There a 3 sources for configuration:
+
+* The default configuration
+* Overrides on the site level, set is set by the initialize call and will  be applied for all diagrams in the site/app. The term for this is the **siteConfig**.
+* Directives - diagram authors can update select configuration parameters directly int he diagram code via directives and these are applied to the render config.
+
+**The render config** is configration that is used when rendering by applying these configurations.
+
+## Theme configuration
+
+## Starting mermaid
+
+```mermaid
+sequenceDiagram
+	Site->>mermaid: initialize
+	Site->>mermaid: content loaded
+	mermaid->>mermaidAPI: init
+```
+
+## Initialize
+
+Intialize call is calloed **only once**. It is called by the site integrator in order to override the default configuration on a site level.
+
+## configApi.reset
+
+This method resets the configuration for a diagram to the site configuration, the configuration provided by the site integrator. Before each rendering of a diagram reset is called in the very beginning of render.
+
+##