Hosting guides still suggest using Yarn

[uncenter]

Lots of the hosting guides still suggest using Yarn for package manager commands, but the main umami repository has switched to using pnpm and uses a pnpm lockfile. I think it would make sense to update these instances.

• docs/content/docs/guides/running-on-neon.mdx

  Line 18 in 6c5dd1c

  5. You should now be able to check the database connection and update the schema (`yarn run build-db && yarn run update-db`).

• docs/content/docs/guides/running-on-forge.mdx

  Lines 28 to 31 in 6c5dd1c

  	```shell
  	yarn install
  	yarn build
  	```

• docs/content/docs/guides/running-on-netlify.mdx

  Line 20 in 6c5dd1c

  4. Change build command to **yarn run build**.

• docs/content/docs/guides/running-on-planetscale.mdx

  Line 17 in 6c5dd1c

  4. Run `yarn run build-db && yarn run update-db`