Add repository README.md to generate_parameter_library package docs

[ottojo]

This makes the README.md appear on the package docs landing page of generate_parameter_library.

rosdoc2 automatically includes a README.md on the package docs landing page (https://docs.ros.org/en/kilted/p/generate_parameter_library/index.html), but the readme is one level up at the repo root in this repository.
Including it in a custom index.rst does not work easily, since rosdoc2 does some wrapping of the package directory, and does not build the docs in-tree. But symlinks seem to work just fine (the link gets resolved when copying standard doc files to the "wrapped" directory). There are certainly ways to improve this (adding the sections to the TOC on the left, for example), but i think this is a good first improvement.

fixes #285

Screenshots:

[christophfroehlich]

What do you think about just moving the readme file into the repository, and adding a different one at the repo, which lists CI stats etc?

[ottojo]

What do you think about just moving the readme file into the repository, and adding a different one at the repo, which lists CI stats etc?

I think at the moment, most people would look at the repo before looking at the package docs, so i'm hesitant to remove any information from there (or add another layer of redirect/link). In would like that in principle, but i think that would make the user experience worse at this time.

[christophfroehlich]

I agree that the published docs are not that visible maybe, but a link in the first paragraph should people also point to the correct page. If people don't read that then I don't have any hope that they would otherwise.
If we branch somewhen (#294 for example), then we will need to point people to the correct branch anyways, even before they start reading docs.

Just my opinion, I'm also fine with this fix by now.