Move test plan near other docs

Author: charisk

…docs/images/highlighted-code-snippet.png docs/images/highlighted-code-snippet.pngextensions/ql-vscode/docs/images/highlighted-code-snippet.png renamed to docs/images/highlighted-code-snippet.png extensions/ql-vscode/docs/images/highlighted-code-snippet.png renamed to docs/images/highlighted-code-snippet.png

@@ -1,6 +1,6 @@
 # Releasing (write access required)
 
-1. Go through [our test plan](/extensions/ql-vscode/docs/test-plan.md) to ensure that the extension is working as expected.
+1. Go through [our test plan](./test-plan.md) to ensure that the extension is working as expected.
 1. Double-check the `CHANGELOG.md` contains all desired change comments and has the version to be released with date at the top.
     * Go through all recent PRs and make sure they are properly accounted for.
     * Make sure all changelog entries have links back to their PR(s) if appropriate.