docs: add cross-references between npx and npm exec

Author: Delapouite

docs/content/commands/npm-exec.md

@@ -388,3 +388,4 @@ project.
 * [npm stop](/commands/npm-stop)
 * [npm config](/commands/npm-config)
 * [npm workspaces](/using-npm/workspaces)
+* [npx](/commands/npx)

docs/content/commands/npx.md

@@ -175,3 +175,4 @@ This resulted in some shifts in its functionality:
 * [npm restart](/commands/npm-restart)
 * [npm stop](/commands/npm-stop)
 * [npm config](/commands/npm-config)
+* [npm exec](/commands/npm-exec)