Add topic tags

[Beliavsky]

I suggest adding topics such as linear-algebra, cuda, blas, lapack in the About section, as explained at https://docs.github.com/en/repositories/managing-your-repositorys-settings-and-features/customizing-your-repository/classifying-your-repository-with-topics

[rasolca]

Thanks @Beliavsky for the suggestion. @msimberg can you please check if I didn't miss any relevant topic?

[msimberg]

@rasolca looks good! In pika we have the following tags:

• cplusplus
• cpp
• gpu
• concurrency
• parallelism
• mpi
• cuda
• hip
• rocm
• stdexec
• p2300

It might at least make sense to add mpi and possibly p2300/stdexec?

[rasolca]

Done.