docs(fluid-dynamics): add fluid API map

[Robby955]

Motivation

Related to #1414, the tracking map for the fluid API in #887. An API map records the implemented status and source location of each requirement for an API, so the gap between a planned API and the current library stays visible in-tree.

Changes

Adds Physlib/FluidDynamics/API-map.yaml.

The completed entries cover what exists: the mass flux density momentumDensity and the equation of continuity in its conservative, residual and smooth forms with the implication between them. The fluid data structure is recorded as not done, since FluidState currently provides density and velocity but the requirement also asks for pressure. The Euler equation, ideal fluid, entropy flux, Bernoulli equation, energy flux and flow out of a volume are not implemented and carry location N/A.

References cite Landau and Lifshitz, Fluid Mechanics, vol. 6, 2nd ed., Chapter 1.

Testing

Ran the API-map linter:

python3 scripts/api_map_linter.py --repo .

Every declared source file and name resolves; no completed entry is missing a location.

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[jstoobysmith]

This will likely need an update based on Physlib#1125, which I will merge now.