[docs] Mark localhost URLs as code snippets

[marcysutton]

To be consistent in the Gatsby docs, we want to mark all localhost URL references as code highlighted strings instead of links and document this preference in the Gatsby Style Guide.

This is in response to PR #19067, which changed some of these references into links; the problem is that unless you're running gatsby develop or gatsby build locally, those links will not work. We'd therefore prefer to have these listed as code blocks, so we don't have invalid links throughout the docs.

This should be documented in the Style Guide. We could add a new section under "Use clear hyperlinks" that says "Mark localhost URLs as code strings" or something along those lines. https://www.gatsbyjs.org/contributing/gatsby-style-guide

[bball07]

PR: [docs] Mark localhost URLs as code snippets #20899

[muescha]

more PR for this issue:

• fix(docs): fix links to localhost (on pixabay tutorial) #21177 fix(docs): fix links to localhost (on pixabay tutorial)
• fix(docs): fix links to localhost, rewording (on strapi blog post) #21178 fix(docs): fix links to localhost, rewording (on strapi blog post)
• fix(docs): fix links to localhost on docs/blog/tutorials ... #21179 fix(docs): fix links to localhost on docs/blog/tutorials ...

[marcysutton]

I think this issue can be closed now, thanks for the watchful eye and PRs @muescha!