docs: show icons in code-groups and code titles, update sidebar

[sheremet-va]

Description

I saw that Vite docs now shows icons in code titles and liked it 😄 This PR is adapted from vitejs/vite#18132

This PR also has a few cosmetic changes:

• The sidebar now always has API and the config links
• The config page now has the /config/file content with examples inside a spoiler. the /config/file should redirect to /config/

Please don't delete this checklist! Before submitting the PR, please make sure you do the following:

• It's really useful if your PR references an issue where it is discussed ahead of time. If the feature is substantial or introduces breaking changes without a discussion, PR might be closed.
• Ideally, include a test that fails without this PR but passes with it.
• Please, don't make changes to pnpm-lock.yaml unless you introduce a new test example.

Tests

• Run the tests with pnpm test:ci.

Documentation

• If you introduce new functionality, document it. You can run documentation with pnpm run docs command.

Changesets

• Changes in changelog are generated from PR name. Please, make sure that it explains your changes in an understandable manner. Please, prefix changeset messages with feat:, fix:, perf:, docs:, or chore:.

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