Update doc pages and docstrings (#200)

Author: abhro

docs/src/index.md

@@ -1,5 +1,23 @@
 # Optimisers.jl
 
+Optimisers.jl defines many standard gradient-based optimisation rules, and tools for applying them to deeply nested models.
+
+This was written as the new training system for [Flux.jl](https://github.com/FluxML/Flux.jl) neural networks,
+and also used by [Lux.jl](https://github.com/LuxDL/Lux.jl).
+But it can be used separately on any array, or anything else understood by [Functors.jl](https://github.com/FluxML/Functors.jl).
+
+## Installation
+
+In the Julia REPL, type
+```julia
+]add Optimisers
+```
+
+or
+```julia-repl
+julia> import Pkg; Pkg.add("Optimisers")
+```
+
 ## An optimisation rule
 
 A new optimiser must overload two functions, [`apply!`](@ref Optimisers.apply!) and [`init`](@ref Optimisers.init).
@@ -38,7 +56,6 @@ state for every trainable array. Then at each step, [`update`](@ref Optimisers.u
 to adjust the model:
 
 ```julia
-
 using Flux, Metalhead, Zygote, Optimisers
 
 model = Metalhead.ResNet(18) |> gpu  # define a model to train
@@ -54,7 +71,6 @@ end;
 
 state_tree, model = Optimisers.update(state_tree, model, ∇model);
 @show sum(model(image));  # reduced
-
 ```
 
 Notice that a completely new instance of the model is returned. Internally, this
@@ -91,7 +107,6 @@ Beware that it has nothing to do with Zygote's notion of "explicit" gradients.
 identical trees of nested `NamedTuple`s.)
 
 ```julia
-
 using Lux, Boltz, Zygote, Optimisers
 
 lux_model, params, lux_state = Boltz.resnet(:resnet18) |> gpu;  # define and initialise model
@@ -113,7 +128,6 @@ opt_state, params = Optimisers.update!(opt_state, params, ∇params);
 
 y, lux_state = Lux.apply(lux_model, images, params, lux_state);
 @show sum(y);  # now reduced
-
 ```
 
 Besides the parameters stored in `params` and gradually optimised, any other model state
@@ -297,7 +311,7 @@ similarly to what [`destructure`](@ref Optimisers.destructure) does but without
 concatenating the arrays into a flat vector.
 This is done by [`trainables`](@ref Optimisers.trainables), which returns a list of arrays:
 
-```julia
+```julia-repl
 julia> using Flux, Optimisers
 
 julia> model = Chain(Dense(2 => 3, tanh), BatchNorm(3), Dense(3 => 2));

src/destructure.jl

@@ -38,7 +38,7 @@ This is what [`destructure`](@ref Optimisers.destructure) returns, and `re(p)` w
 new parameters from vector `p`. If the model is callable, then `re(x, p) == re(p)(x)`.
 
 # Example
-```julia
+```julia-repl
 julia> using Flux, Optimisers
 
 julia> _, re = destructure(Dense([1 2; 3 4], [0, 0], sigmoid))

src/trainables.jl

@@ -32,10 +32,10 @@ julia> trainables(x)
 1-element Vector{AbstractArray}:
  [1.0, 2.0, 3.0]
 
- julia> x = MyLayer((a=[1.0,2.0], b=[3.0]), [4.0,5.0,6.0]);
+julia> x = MyLayer((a=[1.0,2.0], b=[3.0]), [4.0,5.0,6.0]);
 
- julia> trainables(x) # collects nested parameters
- 2-element Vector{AbstractArray}:
+julia> trainables(x) # collects nested parameters
+2-element Vector{AbstractArray}:
  [1.0, 2.0]
  [3.0]
 ```