[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>

plugins/techdocs-backend/config.d.ts

@@ -66,6 +66,13 @@ export interface Config {
     publisher?:
       | {
           type: 'local';
+
+          local?: {
+            /**
+             * Directory to store generated static files.
+             */
+            publishDirectory?: string;
+          };
         }
       | {
           type: 'awsS3';