fix: use relative doc link, add docs URL to error message

- Use relative markdown link for egress reference in filters.md
- Include docs URL in bearerinjector error log for easier debugging

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: mvanhorn

docs/reference/filters.md

@@ -3464,7 +3464,7 @@ It is a special form of `setRequestHeaderFromSecret` with `"Authorization"` head
 `"Bearer "` prefix and empty suffix.
 
 The token file must be located in a directory passed via the `-credentials-paths` flag.
-See the [egress reference](https://opensource.zalando.com/skipper/reference/egress/#example-bearer-injection)
+See the [egress reference](egress.md#example-bearer-injection)
 for a complete configuration example.
 
 Example:

filters/auth/bearer.go

@@ -53,7 +53,7 @@ func newBearerInjectorFilter(s string, sr secrets.SecretsReader) *bearerInjector
 func (f *bearerInjectorFilter) Request(ctx filters.FilterContext) {
 	b, ok := f.secretsReader.GetSecret(f.secretName)
 	if !ok {
-		log.Errorf("Secret %q not found for bearerinjector filter. Make sure the file exists in a directory passed via -credentials-paths flag.", f.secretName)
+		log.Errorf("Secret %q not found for bearerinjector filter. Make sure the file exists in a directory passed via -credentials-paths flag. See https://opensource.zalando.com/skipper/reference/egress/#example-bearer-injection", f.secretName)
 		return
 	}
 	ctx.Request().Header.Set(authHeaderName, authHeaderPrefix+string(b))