_includes: add Cloudflare's base.html

_includes/base.html

@@ -0,0 +1,11 @@
+<!-- Cloudflare recommends these for IPFS at https://developers.cloudflare.com/distributed-web/ipfs-gateway/updating-for-ipfs/ -->
+
+{% assign depth = page.url | split: '/' | size | minus: 1 %}
+{% if    depth <= 1 %}{% assign base = '.' %}
+{% elsif depth == 2 %}{% assign base = '..' %}
+{% elsif depth == 3 %}{% assign base = '../..' %}
+{% elsif depth == 4 %}{% assign base = '../../..' %}{% endif %}
+
+<!-- if anything has to link to root, it should use `{%- include base.html -%}`
+     and normal `/css/main.css` etc. should become `{{base}}/css/main.css`
+     says the page above. base.html ends here. -->