suggestions for documentation improvement

Just collecting some things I found confusing that I plan to make one or more PRs to address in the future

[x] docs for oapply_typed do not say what the purpose of the 3rd argument (Vector{Symbol}) is; update docstring
[x] extra r at start of add_reflexives docstring prevents it from working
[x] it would be nice to describe somewhere what an open PN is and at least the very basics of how to work with the multicospans one gets from using them
[x] it would be nice to show how to combine 2 UWDs into a larger UWD using the categorical interface (probably just implement this https://algebraicjulia.github.io/AlgebraicDynamics.jl/stable/examples/Ecosystem/#Rabbits,-foxes,-and-hawks)
[x] improve the main README.md file to give more information, a few pictures, a link to the Royal Society paper, maybe relevant ACT conf recordings, and background (need to discuss this with the maintainer)
[x] typed product of sird_model and mask_model in the stratification tutorial gives an incorrect model
[x] is there a way to improve the construction of the "mask-vax model" UWD (https://algebraicjulia.github.io/AlgebraicPetri.jl/stable/generated/covid/stratification/#Mask-Vax-Model)? It exhibits many of the problems that the categorical approach hopes to avoid (tedious code with only minor changes between lines).
[x] expand the ecological examples to include the lotka-volterra equations for n species: https://en.wikipedia.org/wiki/Competitive_Lotka%E2%80%93Volterra_equations

AlgebraicJulia / AlgebraicPetri.jl