docs: add info about location.href

Summary

• add section about location.href is not available in SSR

Motivation

some mistakes are coming from using location.href in server side rendering - but there is no place in docs where is a hint about this

-const href = location.href
+const href = `${siteUrl}${pathname}`

References

https://css-tricks.com/how-to-the-get-current-page-url-in-gatsby/#article-header-id-4

Steps to resolve this issue

Docs

maybe placing infos here:

• https://www.gatsbyjs.org/docs/troubleshooting-common-errors/
• https://www.gatsbyjs.org/docs/location-data-from-props/

Draft the doc

• Write the doc, following the format listed in these resources:
  • Overview on contributing to documentation
  • Docs Templates
  • Example of a similar article
• Add the article to the docs sidebar under the [parent doc] section.

Open a pull request

• Open a pull request with your work including the words “closes #[this issue’s number]” in the pull request description

[pieh]

On top of documention, we can make trying to access location.href to throw error in code (which would contain link to appropriate docs. Alternatively we could maybe construct .href for SSR? But I feel like constructing it ourselves might just hide some problems and introduce even more problems (subtle, and hard to debug)

[ehrencrona]

To provide an error message in this situation, there needs to be a change to reach/router. I’ve submitted an issue and a PR on reach that should fix it.