docs: integrations: yarn & pnpm

[mrienstra]

Changes

• Ensures all packages/integrations/*/README.md files contain yarn & pnpm astro add commands.

Testing

Docs-only change.

Docs

Docs-only change.

Other

Search:

```bash\nnpx astro add (.+)\n```

Replace:

```sh
# Using NPM
npx astro add $1
# Using Yarn
yarn astro add $1
# Using PNPM
pnpm astro add $1
```

Find occurrences where commands for all 3 package managers are provided:

```sh\n# Using NPM\nnpx astro add .+\n# Using Yarn\nyarn astro add .+\n# Using PNPM\npnpm astro add .+\n```

(returned 14 results before the changes in this PR, 19 results after)

[changeset-bot]

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Latest commit: 6ce7967

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[sarah11918]

This looks good, thanks @mrienstra