Use pdoc rather than pdoc3

**Is your feature request related to a problem? Please describe.**
[pdoc](https://pdoc.dev/) and [pdoc3](https://pdoc3.github.io/pdoc/) are both continuations of a discontinued pdoc project. The two packages are conflicting. At present, mbed-tools uses pdoc3, however pdoc is a larger and more maintained library, also [pdoc3 is the subject of some controversy](https://github.com/pdoc3/pdoc/issues/87).

**Describe the solution you'd like**
Change the project to depend on pdoc rather than pdoc3. As far as I can tell, this should be a near drop-in replacement, with just a handful of `pdoc3`s needing to be changed to `pdoc`s.

**Describe alternatives you've considered**
The installation instructions say
> We recommend installing mbed-tools in a Python virtual environment to avoid dependency conflicts.

Doing so mitigates this issue, but I think installation should be made as easy as possible without requiring a virtual environment.

Inspect the dependency declarations and installation instructions for references to pdoc3, then search the repository for the remaining package name. Replace the dependency and references with pdoc, and verify that mbed-tools can be installed without the conflicting package and that its documentation generation still works.

Written by the indexing model from the issue text.