[doc,feat]: Add proper documentation and simplify autoposting

[null8626]

The following pull request does the following:

• Add documentation with Doxygen.
• Simplify autoposting by making developers just use the start_autoposter() method.
• Replace dpp::async<topgg::result<T>> in coroutine return types with topgg::async_result<T>.
• Default to C++20 if DPP_CORO is enabled, C++17 otherwise.
• Remove the unnecessary topgg::base_client abstract class.
• Remove over-dependence on macros.
• Add resource file containing library metadata for Windows.

[null8626]

cc @ffamilyfriendly

[ffamilyfriendly]

LGTM