feat: pathname in SSR APIs when in develop

[neo]

Update the docs for this change: #9792.
It's not a breaking change.
It also adds the pathname to onRenderBody and onPreRenderHTML in developer.

[neo]

Actually I also found that it only works in build but not develop?

[pieh]

In develop it will always be / (because we don't create actual html pages there other than main one), but it probably makes sense to use it anyway for parity

[neo]

good point, I added that because it broke npm start when I tried to use pathname in the SSR APIs.

actually I was also confused that why SSR was called during gatsby develop...

[pieh]

Thanks @neo!