docs: improve info about linux native build of the sdk

[HKayn]

What started as simply fixing some spelling errors lead to me doing a complete pass over the doc page. I hope this isn't too much.

This PR does the following on For-Game-Developers.md:

• Improve spelling, grammar & punctuation
• Adjust wording to achieve a more professional tone
• Align vocabulary with that of the GOG Dev Docs (e.g. "user identity" -> "player authentication")
• Add links to listed games & sort the list of games alphabetically
• Reword the bullet points in "Why bother?" to more strongly emphasize the benefits of using the Steam-runtime library
• Rearrange the order of numbered list items in "Why bother?" to allow the Multiplayer topic to lead fluidly into the introduction to Comet.

Requesting a review on the accuracy of the few technical details I've added.