added repeater grid example and started how-to tutorial with animation

docs/src/howto/repeater_grid/repeater_grid.md

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+# [Entanglement Generation On A Repeater Grid](@id Entanglement-Generation-On-A-Repeater-Grid)
+
+```@meta
+DocTestSetup = quote
+    using QuantumSavory
+end
+```
+
+This section provides a detailed walkthrough of how the QuantumSavory.jl can be used to simulate entanglement genearation on a network of repeaters in a square grid topology.
+

examples/repeater_grid/repeater_grid.jl

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+using QuantumSavory
+
+# For Simulation
+using ResumableFunctions
+using ConcurrentSim
+using QuantumSavory.ProtocolZoo
+using Graphs
+
+# For Plotting
+using GLMakie
+GLMakie.activate!(inline=false)
+using NetworkLayout
+
+## Custom Predicates
+
+function check_nodes(net, c_node, node; low=true)
+    n = Int(sqrt(size(net.graph)[1])) # grid size
+    c_x = c_node%n == 0 ? c_node ÷ n : (c_node ÷ n) + 1
+    c_y = c_node - n*(c_x-1)
+    x = node%n == 0 ? node ÷ n : (node ÷ n) + 1
+    y = node - n*(x-1)
+    return low ? (c_x - x) >= 0 && (c_y - y) >= 0 : (c_x - x) <= 0 && (c_y - y) <= 0
+end
+
+# functions for picking the furthest node
+function distance(n, a, b)
+    x1 = a%n == 0 ? a ÷ n : (a ÷ n) + 1
+    x2 = b%n == 0 ? b ÷ n : (b ÷ n) + 1
+    y1 = a - n*(x1-1)
+    y2 = b - n*(x2-1)
+
+    return x1 - x2 + y1 - y2
+end
+
+function choose_node(net, node, arr; low=true)
+    grid_size = Int(sqrt(size(net.graph)[1]))
+    return low ? argmax((distance.(grid_size, node, arr))) : argmin((distance.(grid_size, node, arr)))
+end
+
+## Simulation
+
+n = 6
+
+graph = grid([n,n])
+
+for i in 1:(n^2 - n + 1) # add diagonal channels
+    if !iszero(i%n) # no diagonal channel from last node in a row
+        add_edge!(graph, i, i + n + 1)
+    end
+end
+
+net = RegisterNet(graph, [Register(8) for i in 1:n^2])
+
+sim = get_time_tracker(net)
+
+for (;src, dst) in edges(net)
+    eprot = EntanglerProt(sim, net, src, dst; rounds=5, randomize=true) # A single round doesn't always get the ends entangled, when number of nodes is high
+    @process eprot()
+end
+
+for i in 2:(size(graph)[1] - 1)
+    l(x) = check_nodes(net, i, x)
+    h(x) = check_nodes(net, i, x; low=false)
+    cL(arr) = choose_node(net, i, arr)
+    cH(arr) = choose_node(net, i, arr; low=false)
+    swapper = SwapperProt(sim, net, i; nodeL = l, nodeH = h, chooseL = cL, chooseH = cH, rounds = 5) # A single round doesn't always get the ends entangled, when number of nodes is high
+    @process swapper()
+end
+
+for v in vertices(net)
+    tracker = EntanglementTracker(sim, net, v)
+    @process tracker()
+end
+
+## Visualization
+layout = SquareGrid(cols=:auto, dx=10.0, dy=-10.0)(graph)
+fig = Figure(resolution=(600, 600))
+_, ax, _, obs = registernetplot_axis(fig[1,1], net;registercoords=layout)
+
+display(fig)
+
+step_ts = range(0, 10, step=0.1)
+record(fig, "grid_sim6x6.mp4", step_ts; framerate=10, visible=true) do t
+    run(sim, t)
+    notify(obs)
+end