Add a logo and open the README with a centered header of logo, name, tagline, links and badges

[avgvi]

Gives the project a logo and opens the README with a centered header in the style of the other community forks.

• docs/_static/: the lockup (icon plus wordmark and "COMMUNITY") and the icon alone, each in a light and a dark variant as SVG with the text converted to outlines (Space Grotesk SemiBold and IBM Plex Mono, SIL Open Font License), plus 1200-pixel PNG exports of the lockups, a 64-pixel favicon and a 240-pixel icon.
• README: centered header with the lockup switching between light and dark through a picture element, the project name, the tagline, a [Tutorial] - [API] - [Docs] - [Changelog] row and the badges, followed by the unchanged fork alert and body. The images load from master, so the header shows them once this is merged.
• Docs: the lockup in the sidebar instead of the project name, the icon as favicon, and an explicit title on the API page so the landing page lists nine pages instead of every API section.

Verified: Sphinx with warnings as errors, doc8, twine check --strict on the sdist, and GitHub's Markdown renderer keeping the picture element and the five badges.