docs: clarify how activation works in practice

[stinos]

Description

Add basic examples of pixi shell-hook usage since those seemed to be missing so far and make it clear that pixi shell should never be put in a shell's startup file.

Fixes part of #5510.

Note: theare are also other documents touching on activation, like https://pixi.prefix.dev/latest/workspace/environment/#activation and https://pixi.prefix.dev/latest/advanced/pixi_shell/, but I left those as-is because imo this basic yet important information should be put where it is most likely to be looked for i.e. next to the command's own explanation.

On the other hand something like https://pixi.prefix.dev/latest/advanced/pixi_shell/#issues-with-pixi-shell is almost about the exact same change I made in shell.md so please advise if this needs to be structured in a different way.
Perhaps simply linking directly to the relevant .md files in the aformentiond links would already help? It's a bit weird to have a page dedicated to shell without actually linking to its core documentation?

How Has This Been Tested?

I only looked at the docs in a markdown previewer: don't know how else to check.

[baszalmstra]

Thanks!