chore: card grids on homepage

[HonkingGoose]

Changes:

• Use the newly unlocked card grids feature from Material for MkDocs [^1]
• Link to Renovate's licence file on the renovatebot/renovate repository
• Put the card grid in a prettier-ignore block, to prevent Prettier from messing up the formatting

Context:

Break up the wall-of-text with a card grid for the main Renovate features. I've added icons and flavor text. I did not need to change the mkdocs.yml config file, because everything we need is already enabled.

[^1]: Material for MkDocs, card grids

[HonkingGoose]

Looks like I'm hitting some kind of rate-limit in the docs building part of the devcontainer / devcontainer-test (pull_request) test. Quote from the latest test results:

  > renovate@0.0.0-semantic-release release:prepare /workspaces/renovatebot.github.io/deps/renovate
  > ts-node tools/generate-docs.ts

   INFO: generating docs

   INFO: * static

   INFO: * fetching open GitHub issues

  ERROR: Error getting query results
         "err": {
           "message": "rate-limit-exceeded",
           "stack": "Error: rate-limit-exceeded\n    at handleGotError (/workspaces/renovatebot.github.io/deps/renovate/lib/util/http/github.ts:107:12)\n    at GithubHttp.request (/workspaces/renovatebot.github.io/deps/renovate/lib/util/http/github.ts:370:13)\n    at processTicksAndRejections (node:internal/process/task_queues:95:5)\n    at async GithubHttp.requestJson (/workspaces/renovatebot.github.io/deps/renovate/lib/util/http/index.ts:299:17)\n    at async getOpenGitHubItems (/workspaces/renovatebot.github.io/deps/renovate/tools/docs/github-query-items.ts:53:17)\n    at async /workspaces/renovatebot.github.io/deps/renovate/tools/generate-docs.ts:35:23"
         }

[viceice]

now green

@rarkins please merge or approve 😉