Readme update

[SaschaWillems]

Description

This PR slightly updates the repo's main readme:

• Update and move third party dependencies to a separate readme (we had some missing, and some no-longer used were still referenced)
• Add iOS as supported platform (we added support for it some time ago, but didn't update the readme)
• Slightly adjust linkage, so that the readme shown on github links to the docs site (doesn't affect Antora builds).

Note: Pure documentation fix

General Checklist:

Please ensure the following points are checked:

• My code follows the coding style
• I have reviewed file licenses
• I have commented any added functions (in line with Doxygen)
• I have commented any code that could be hard to understand
• My changes do not add any new compiler warnings
• My changes do not add any new validation layer errors or warnings
• I have used existing framework/helper functions where possible
• My changes do not add any regressions
• I have tested every sample to ensure everything runs correctly
• This PR describes the scope and expected impact of the changes I am making

Note: The Samples CI runs a number of checks including:

• I have updated the header Copyright to reflect the current year (CI build will fail if Copyright is out of date)
• My changes build on Windows, Linux, macOS and Android. Otherwise I have documented any exceptions

[tomadamatkinson]

Much better!

[marty-johnson59]

Merging - 3 approvals