doc: add brackets to implicit markdown links

Author: nschonni

doc/api/n-api.md

@@ -1805,7 +1805,7 @@ Returns `napi_ok` if the API succeeded.
 
 This API allocates a JavaScript value with external data attached to it. This
 is used to pass external data through JavaScript code, so it can be retrieved
-later by native code using [`napi_get_value_external`].
+later by native code using [`napi_get_value_external`][].
 
 The API adds a `napi_finalize` callback which will be called when the JavaScript
 object just created is ready for garbage collection. It is similar to

doc/api/util.md

@@ -1767,7 +1767,7 @@ added: v0.7.5
 deprecated: v6.0.0
 -->
 
-> Stability: 0 - Deprecated: Use [`Object.assign()`] instead.
+> Stability: 0 - Deprecated: Use [`Object.assign()`][] instead.
 
 * `target` {Object}
 * `source` {Object}

doc/releases.md

@@ -660,7 +660,7 @@ To ensure communication goes out with the timing of the blog post, please allow
 will be shared with the community in the email to coordinate these
 announcements.
 
-Ping the IRC ops and the other [Partner Communities] liaisons.
+Ping the IRC ops and the other [Partner Communities][] liaisons.
 
 ### 19. Celebrate