IIS Reverse Proxy: Add preserveHostHeader step, + add extra examples

[IllusionVK]

• Added preserveHostHeader step to remove origin errors

• The primary example of localhost:8080 has been changed to yourdomain.com:8080 to both provide an example for an external domain, but also to reduce opportunity for readers to get confused about how the reverse proxy works, so they can see it can be used for both internal and external websites

• Added extra emphasis to IIS Config steps by labelling them Step 1 and Step 2, because somehow we managed to miss step 1 when configuring our site.

[achatterjee-grafana]

Adding @marcusolsson and @zuchka for their review, as this is Grafana tutorial content.

[zuchka]

thank you so much for all this hard work, @IllusionVK!

our users will really benefit from this ❤️

[IllusionVK]

Hey @zuchka Matt, thanks for your positivity! Appreciated 👍