Add Fastly configuration rst

[DemigodCode]

First version of a fastly configuration example.

[dbu]

thanks a lot! i have some tweaks

[dbu]

dang, there seems to be quite some code rot in the 2.x branch :-(

at leat the spellcheck build is green. i will have a look into the failing tests separately, they have nothing to do with this PR.

[dbu]

if you could rebase on 2.x branch, the readthedocs build will work again and we can check in the preview if the new documentation page looks right.

[dbu]

the preview makes me realize: we need to add fastly in the proxy-configuration.rst page to have it show up in the table of content.

https://foshttpcache--547.org.readthedocs.build/en/547/proxy-configuration.html (the stylesheet seems to be missing in the preview, thats why the page looks a bit strange - it is only applied once the changes are merged)

[DemigodCode]

Yeah just realized the configuration toctree thing. Already did a push.

[dbu]

awesome, thanks a lot for this contribution!