Improve accessibility for screenreaders

[HonkingGoose]

Describe the proposed change(s).

Todo:

• docs(readme): better alt text, add toggleable list of companies/projects that use Renovate #29022
• docs(homepage): better alt text, list companies that use Renovate #29345
• docs(homepage): convert Markdown list to plain HTML list #29369
• docs(onboarding): better alt text for images #29347
• docs(dependency pinning): better alt text for images #29350
• docs(swissquote): better alt text for images #29351
• Check that all images have a good alt text, add/edit if needed
• Check that the docs use good link names
• Better link name in PR body text in the footer #26164