Add a structured WaterLily organization profile README

[Copilot]

This updates the WaterLily organization profile to present the ecosystem in a structured, scannable way, following the grouped-repository style used by SpeedyWeather. It replaces the current placeholder state with an organization-facing README that helps visitors quickly understand what each repository is for.

• Organization profile

  • Add profile/README.md as the public organization profile entrypoint.
  • Introduce a short top-level summary of WaterLily and its scope.

• Repository grouping

  • Organize repositories into clear sections:
    • core projects
    • geometry / boundary conditions / visualization
    • examples / tutorials / performance
    • papers / talks
    • community

• Repository catalog

  • Add concise descriptions for the main repositories, including:
    • WaterLily.jl
    • GeometricMultigrid.jl
    • LilyPad.jl
    • ParametricBodies.jl
    • WaterLilyMeshBodies.jl
    • BiotSavartBCs.jl
    • Pathlines.jl
    • WaterLily-Examples
    • WaterLily-Workshop
    • WaterLily-Benchmarks
    • WaterLily.jl_CPC_2024
    • JuliaCon2024

• Discovery links

  • Add direct links to the main documentation site and the organization’s .github repository for shared profile/community files.

Original prompt

Create a WaterLily organisation README, following the structure of the one in https://github.com/SpeedyWeather, where you list the different repositories, explain their content, classify them in different groups, etc