docs/index.html: various tweaks.
docs/index.html: various tweaks.

* add missing `alt` tags
* add image dimensions
* add `rel=noopener` for external links
* remove unneeded end tags since hTML doesn't require it
* use lowercase `doctype` since it compresses slightly better
* fix HTML validation errors

taniarascia/new-moon,taniarascia/new-moon