fix(snipsync): restrict target file scanning to .md/.mdx

snipsync.config.yaml never set features.allowed_target_extensions,
which snipsync defaults to [] ("scan every file under targets/docs").
Its Sync.js reads every scanned file line-by-line as UTF-8 text and
rewrites it the same way, which silently corrupts binary assets
(PNG/SVG) with non-UTF-8 bytes on every daily sync run.

Confirmed on PR #4908: a synced PNG grew from 1.1MB to 2.1MB and is
no longer a valid image (each non-UTF-8 byte, e.g. PNG's 0x89 magic
byte, was replaced with the 3-byte U+FFFD replacement character).

Restricting the scan to .md/.mdx (the only content types docs/
snippets live in) avoids touching the 6 PNGs and 3 SVGs in docs/.

Author: Duncanma

snipsync.config.yaml

@@ -48,3 +48,8 @@ targets:
 features:
   enable_source_link: true
   enable_code_block: true
+  # Without this, snipsync scans every file under `targets` (docs/), including
+  # binary assets. It reads/writes files line-by-line as UTF-8 text, which
+  # corrupts any PNG/SVG in its path (invalid UTF-8 bytes become U+FFFD).
+  # See: https://github.com/temporalio/documentation/pull/4908
+  allowed_target_extensions: [.md, .mdx]