message: docs: tweak catalog section in threat model and add proxy section
diff: @@ -54,7 +54,7 @@ Backstage also supports authentication through a proxy where the user identity i
  ## Catalog
  -Integrators should configure `catalog.rules` and `catalog.locations[].rules` to limit the allowed entity kinds that users can define, in general it is best to restrict definition of User, Group, and Template entities so that internal users cannot register additional ones. Template entities define actions that are executed on the backend hosts, while the goal is for these actions to be secure regardless of input, it is still a more sensitive context and it is recommended that you protect it with additional checks. It is very important to not allow registration of User and Group entities if you ingest and rely on these as organizational data in your catalog. Doing so could otherwise open up for the ability to impersonate users and confuse group membership information. You should always ingest organizational data using a statically configured catalog location or an entity provider reading from a trusted source.
  +Integrators should configure `catalog.rules` and `catalog.locations[].rules` to limit the allowed entity kinds that users can define. In general it is best to restrict definition of User, Group, and Template entities so that internal users cannot register additional ones. Template entities define actions that are executed on the backend hosts, and while the goal is for these actions to be secure regardless of input, it is still a more sensitive context and it is recommended that you protect it with additional checks. It is very important to not allow registration of User and Group entities if you ingest and rely on these as organizational data in your catalog. Doing so could otherwise open up for the ability to impersonate users and confuse group membership information. You should always ingest organizational data using a statically configured catalog location or an entity provider reading from a trusted source. The entities emitted directly by an entity provider are always trusted and rules are not applied to them, but any entities produced further down the chain are still subject to the rules.
  The Catalog does not aim to protect against resource exhaustion attacks in its default setup. If you need to prevent your internal users from being able to register large amounts of entities, then it is recommended to disable entity registration and use a different approach for discovering entities. One way to mitigate any resource exhaustion attacks is to only allow the catalog to read from trusted SCM sources that have an audit trail. Catalog currently lacks limits for entity hierarchy depth and entity size, which we hope to address in the future.
  @@ -81,3 +81,9 @@ When documentation is generated locally, integrators are responsible for ensurin
  Regardless of backend configuration, the TechDocs frontend does not trust the generated HTML of any documentation sites and therefore applies a strict sanitization process before rendering out any content to users.
  By default, all TechDocs documentation is visible to all Backstage users. Access can be restricted to TechDocs sites by configuring view permissions for the Catalog.
  +
  +## Proxy
  +
  +The proxy backend acts as a utility for frontend plugins to access remote services that may not be set up to receive traffic directly from the Backstage frontend. Typical reasons for this would be that the upstream service does not provide the appropriate CORS headers or does not serve its content over HTTPS.
  +
  +The proxy entries are configured through static configuration. Each entry has a mount path and an upstream target, and also supports other options such as limiting the allowed methods, or injecting additional headers. It is recommended to avoid injecting authentication headers for upstream services, as this is a risky way to decorate requests with credentials. Anyone with access to your Backstage deployment will be able to make requests to the upstream service using the injected credentials. It is recommended that you instead create a backend plugin that forwards individual requests to the upstream service in a secure way. In case you do end up injecting credentials into upstream requests, be sure that you are not exposing any sensitive information or actions. You should also restrict the access as much as possible, for example using the `allowedMethods` option to limit the methods that can be used, and using tokens with the minimum required authorization scope.
  