Refactoring documentation

[SeverinDiederichs]

This is the first PR in a series of PRs to improve the readthedocs documentation.

1. It updates the GH readme. Here, also the HiPACE++ workshop is announced.
2. The HiPACE++ workshop is announced on the RTD doc as well.
3. The general index is improved
4. The parameter section is better ordered, although more ordering is needed.

• Small enough (< few 100s of lines), otherwise it should probably be split into smaller PRs
• Tested (describe the tests in the PR description)
• Runs on GPU (basic: the code compiles and run well with the new module)
• Contains an automated test (checksum and/or comparison with theory)
• Documented: all elements (classes and their members, functions, namespaces, etc.) are documented
• Constified (All that can be const is const)
• Code is clean (no unwanted comments, )
• Style and code conventions are respected at the bottom of https://github.com/Hi-PACE/hipace
• Proper label and GitHub project, if applicable

[MaxThevenet]

Great, thanks for this PR! few comments/typos below, I'll just commit and merge.