[Docs] Add uv (and uvx) as options for installing/running Posting

[mayanyax]

The recent 0.4 release of uv now allows the installation and running of tools like Posting. Just in case you haven't seen it, the uv blog post announcing their release even shows posting as an example of using the new uvx and uv tool commands.

Assuming uv has been installed then there are couple of options:

• You can use uvx posting or uv tool run posting to run it in a temporary environment without actually installing it
• You use uv tool install posting to permanently install it in its own standalone environment

Like rye these commands are very fast (on my laptop, about 590 ms to resolve all 32 dependencies and then about 85 ms to install them).

Both rye and uv projects are under the stewardship of the same organisation. In which case I wonder whether the installation docs might move to recommending uv now instead of rye since that is where all the development activity is and I think only one of the 2 needs mentioning. Obviously this doesn't affect the documentation of the other installation options like pipx etc.