docs: added a private messaging page.

[Jaskowicz1]

This PR creates a new page, teaching people how to private message people using their bot. It also involves the co_mentionable command option, teaching people how to allow mentions as a command option.

I have followed the standard from #820 to adding a new page in interactions_and_components.md, hence why it looks different to all the others as they are under changes from that PR.

• My pull request is made against the dev branch.
• I have ensured that the changed library can be built on your target system. I did not introduce any platform-specific code.
• I have ensured that all methods and functions are fully documented using doxygen style comments.
• I tested my commits, by adding a test case to the unit tests if needed
• I have ensured that I did not break any existing API calls.
• My code follows the coding style guide (if you are not sure, match the code style of existing files including indent style etc).
• I have not built my pull request using AI, a static analysis tool or similar without any human oversight. Where I have generated this pull request using a tool, I have justified why this is needed.

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