Combine overlapping guidelines

[jyasskin]

It's hard to review the document if it makes the same point in many places.

For example,

• https://w3c.github.io/sustyweb/#minify-your-html-css-and-javascript, https://w3c.github.io/sustyweb/#use-code-splitting-within-projects, https://w3c.github.io/sustyweb/#avoid-code-duplication, https://w3c.github.io/sustyweb/#use-html-elements-correctly, https://w3c.github.io/sustyweb/#compress-your-files, and maybe others are about reducing the bytes sent over the wire.
• https://w3c.github.io/sustyweb/#avoid-using-deprecated-or-proprietary-code and https://w3c.github.io/sustyweb/#use-the-latest-stable-language-version are about keeping dependencies up to date.
• https://w3c.github.io/sustyweb/#provide-suitable-alternatives-to-web-assets, https://w3c.github.io/sustyweb/#support-non-graphic-ways-to-interact-with-content, https://w3c.github.io/sustyweb/#develop-a-mobile-first-layout, and https://w3c.github.io/sustyweb/#adapt-to-user-preferences are about tailoring sites to individual user needs.

and probably others can be combined too.