Plugin docs: document the storefront hooks #251

Problem

The plugin docs at docs.bwh.tech/commera do not cover the storefront hooks:

• commera_storefront_include_js and commera_storefront_include_css
• commera_storefront_blocks and its slots cart_banner, checkout_banner, page_overlay
• the per-plugin storefront switch (Commera Settings > Storefront apps, and Settings > Plugins after feat(plugins): switch a plugin's storefront scripts on in Settings > Plugins #244)
• that includes do not load on the checkout page, and the theme's --ls-* CSS variables a plugin can use

The Dashboard entries page also still lists frappe-ui's Dialog under build checks; #242 removes that check.

Proposal

A Storefront page in Plugin reference with the hooks, slots, the switch, the checkout rule, the theme variables and a small example. Remove Dialog from the build-checks table.

Context

Found while building the Buy Links popup, which uses commera_storefront_include_js.