web: repo front matter on the code tab only, tabs wrap on a phone, landing says what the forge does !225

Two of the three items of #134, plus a CONTRIBUTING fix folded in.

• Tabs on a phone. The bar scrolled sideways with its scrollbar hidden, so "Releases" and "Wiki" past the edge looked absent. Under 52rem the tabs wrap to a second row; nothing is clipped and nothing needs a gesture to find.
• Landing page. It was a paragraph and a code block on an empty screen. It now says in three short sections what a reader, a writer and a reviewer can do here, above the same quickstart and explore link; one column on a phone.
• CONTRIBUTING.org still asked for an invite; registration has been open with email verification since v1.7.0. The account section now shows register --username --email and email verify, and points at the web signup.

Not done: compacting the header off the code tab. The first cut hid the description, topics, website and mirrors on the other tabs, and TestReadmeRelativeLinks refused it for a reason that holds: the header sits above the tab bar, so anything shown on one tab and not another moves the navigation between clicks of that navigation. The header stays the same everywhere.

Stacked on !224; no code dependency.

Closes #134