Change all shopify.dev theme doc

[PolMrt]

What are you adding in this PR?

Change all links to theme docs from shopify.dev/docs/themes/* to shopify.dev/docs/storefronts/themes/* as mentioned in #471.

Note: this overlaps with #470 and #468.

What's next? Any followup issues?

N/A

What did you learn?

N/A

Before you deploy

• This PR includes a new checks or changes the configuration of a check
  • I included a minor bump changeset
  • It's in the allChecks array in src/checks/index.ts
  • I ran yarn build and committed the updated configuration files
    • If applicable, I've updated the theme-app-extension.yml config

• I included a minor bump changeset
• My feature is backward compatible

• I included a patch bump changeset

[karreiro]

Thank you, @PolMrt! I've noticed only two links that we need to update.

Also, the content_for_block page doesn't exist yet, but I will take care of creating that page.

Thanks again for this PR. With the updates applied, we will be ready to merge 🚀

[karreiro]

Oh! I've just applied the changes myself :)