fix: make techdocs s3 credentials optional in config schema

Signed-off-by: Andrew Thauer <athauer@wealthsimple.com>

.changeset/bright-taxis-stare.md

@@ -0,0 +1,5 @@
+---
+'@backstage/plugin-techdocs-backend': patch
+---
+
+Make techdocs s3 publisher credentials config schema optional.

plugins/techdocs-backend/config.d.ts

@@ -83,12 +83,12 @@ export interface Config {
                * User access key id
                * @visibility secret
                */
-              accessKeyId: string;
+              accessKeyId?: string;
               /**
                * User secret access key
                * @visibility secret
                */
-              secretAccessKey: string;
+              secretAccessKey?: string;
               /**
                * ARN of role to be assumed
                * @visibility backend