added docs about caching

Author: motatoes

docs/ce/howto/generate-projects.mdx

@@ -67,4 +67,43 @@ generate_projects
         root_dir: "staging/"
         workflow: default
         workflow_file: digger_workflow_staging.yml
-```
+```
+
+# Caching digger config
+
+In cases where there is a very large terragrunt monorepo and the generation takes alot of time you can
+instruct digger to perform caching of the config periodically. Digger will store a cache of the config and you can periodically refresh it from cache to avoid generating on every pull request.
+
+To enable loading cache from pull request you should set this flag:
+
+```
+"DIGGER_CONFIG_REPO_CACHE_ENABLED=1"
+```
+
+ You also need to set the value for "DIGGER_INTERNAL_SECRET" to be a secret that you generate. It will be used for calling the internal endpoint
+ for refreshing from cache.
+
+The internal url to refresh the cache for a repo and a branch is /_internal/update_repo_cache. It expects some arguments.
+Here is a curl request for it as an example"
+
+```
+ curl -XPOST https://mydigger.app/_internal/update_repo_cache \
+-H "Authorization: Bearer $DIGGER_INTERNAL_SECRET" \
+-d '{
+      "repo_full_name": "diggerhq/demo-opentofu",
+      "branch": "main",
+       "installation_id": 56497610
+}'
+```
+
+
+Where repo_full_name is just a path to the repo
+branch is the branch you want to cache from (always this would be your default branch
+installation_id is your github installation ID and can be fetched from your github app settings (the place where you installed the app from)
+
+You should get back a response that the caching has been initiated and in the backend logs you will see logs about cloning. You should see an entry for that repo in the repos_cache table if things are successful.
+Now you can decide how often you want to refresh the cache and you can then set up a cron task to curl that endpoint for the periods that you want to refresh.
+
+
+Once it is cached, all future requests will not regenerate projects but instead it will laad it from the cache. In order to force digger to regenerate in a specific pull request you can add the label "digger:no-cache"
+to it and if added then all digger plan and digger apply actions on that pr will regenerate the projects.