[ZEUS-4449] Improve documentation around the user-agent issue

[artem-y-pamediagroup]

Jira

• https://pa-media-group.atlassian.net/browse/ZEUS-4449

Changes

• updated the README section about playing free and non-free videos in an attempt to describe more cases and make things more explicit
• dedicated a whole subsection to configuring the user-agent header to make clearer and also make it stand out more
• updated existing Swift docs on the Getting Started page: moved the step describing errors into the docs, fixed grammar and a few typos, re-generated the docs
• cleaned up the Swift docs: removed placeholder comments with author and date from the code examples, removed redundant duplication of step names (Step 1: because it says Step 1 right above etc.)
• added an optional Step 2 about the user-agent header as an incremental step, which means you can see difference in the implementation compared to the previous step.