docs: fix broken links and anchors across documentation

Fix ~30 broken links and anchors across the documentation site, including
incorrect relative paths, mismatched anchor names, zero-width characters
in URLs, and references to renamed or removed headings.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Fredrik Adelöw <freben@spotify.com>

docs/features/software-templates/index.md

@@ -20,7 +20,7 @@ See the [Writing Custom Actions guide](./writing-custom-actions.md#naming-conven
 
 ## Prerequisites
 
-These docs assume you have already gone over the [Backstage Getting Started](../../getting-started) section and you are able to run Backstage locally or it has been deployed somewhere.
+These docs assume you have already gone over the [Backstage Getting Started](../../getting-started/index.md) section and you are able to run Backstage locally or it has been deployed somewhere.
 
 ## Getting Started
 

docs/features/software-templates/writing-templates.md

@@ -576,7 +576,7 @@ token from the user, which you can do on a per-provider basis, in case your
 template can be published to multiple providers.
 
 Note, that you will need to configure an [authentication provider](../../auth/index.md#configuring-authentication-providers), alongside the
-[`ScmAuthApi`](../../auth/index.md#scaffolder-configuration-software-templates) for your source code management (SCM) service to make this feature work.
+[`ScmAuthApi`](../../auth/index.md#custom-scmauthapi-implementation) for your source code management (SCM) service to make this feature work.
 
 ### The Repository Branch Picker