Add initial explainer for menu elements (#1181)

* Add initial explainer for menu elements

* reformat + add images

Author: dizhang168

.github/CODEOWNERS

@@ -15,6 +15,7 @@
 /site/src/pages/components/*invokers.explainer.mdx @lukewarlow @keithamus @gregwhitworth @mfreed7 @dbaron
 /site/src/pages/components/*openable.explainer.mdx @lukewarlow @scottaohara @gregwhitworth @mfreed7 @dbaron
 /site/src/pages/components/*focusgroup.explainer.mdx @travisleithead @gregwhitworth @mfreed7 @dbaron
+/site/src/pages/components/menu.explainer.mdx @domfarolino @dizhang168 @josepharhar @mfreed7
 /site/src/pages/components/press-button.explainer.mdx @lukewarlow @scottaohara @gregwhitworth @mfreed7 @dbaron
 /site/src/pages/components/*select.* @josepharhar @mfreed7 @gregwhitworth
 /site/src/pages/components/*selectlist.* @josepharhar @mfreed7 @gregwhitworth