[TechDocs] Added configuration for local publishing target

This patch adds a configuration option for setting the "local"
techdocs target directory. The target directory can be set using
the "techdocs.publisher.local.publishDirectory".

This fixes two "TODOs" in the "plugins/techdocs-node/src/stages
/publish/local.ts" file:

*  Use a more persistent storage than node_modules or /tmp directory.
   Make it configurable with techdocs.publisher.local.publishDirectory
*  Move the logic of setting staticDocsDir based on config over to
   fromConfig, and set the value as a class parameter.

Signed-off-by: Niklas Aronsson <niklasar@axis.com>

docs/features/techdocs/configuration.md

@@ -74,6 +74,12 @@ techdocs:
 
     type: 'local'
 
+    # Optional when techdocs.publisher.type is set to 'local'.
+
+    local:
+      # (Optional). Set this to specify where the generated documentation is stored.
+      publishDirectory: '/path/to/local/directory'
+
     # Required when techdocs.publisher.type is set to 'googleGcs'. Skip otherwise.
 
     googleGcs: