Put doctests in test suite

[abhro]

Run doctests as part of test suite

[abhro]

skipmissings throws up deprecation warnings, not sure how to handle that yet. Doctests won't pass until that's been dealt with.

[abhro]

Tried the jldoctest filters, they don't seem to work for some reason. Attempted patch:

diff --git a/src/Missings.jl b/src/Missings.jl
index 35cfd18..a17bd02 100644
--- a/src/Missings.jl
+++ b/src/Missings.jl
@@ -237,7 +237,8 @@ skipping elements at positions where at least one of the iterators returns `miss
 (listwise deletion of missing values).

 # Examples
-```jldoctest
+
+```jldoctest; filter = r".*Warning:.*\\n.*\\n.*"
 julia> x = [1, 2, missing, 4]; y = [1, 2, 3, missing];

 julia> tx, ty = skipmissings(x, y);
@@ -464,7 +465,7 @@ to `skipmissing` and elements for which `f` returns `false`. This method
 only applies when all iterators passed to `skipmissings` are arrays.

 # Examples
-```jldoctest
+```jldoctest; filter = r".*Warning:.*\\n.*\\n.*"
 julia> x = [missing; 2:9]; y = [2:9; missing];

 julia> mx, my = skipmissings(x, y);

[abhro]

I'm just gonna turn doctests off for those two particular blocks, maybe that'll help.

[abhro]

Added compat for Documenter v0.27 so that Pkg resolve doesn't fail on Julia 1.0. Note that the doctests only run on Julia v1.6+, so this isn't even an issue because we'll not be using Documenter, but since we can't add packages conditionally based on Julia version (AFAIK), this stops the CI from erroring out where it installs an unnecessary dependency of Documenter.

[abhro]

Now the Windows CI test fails because of different word size on x86. 🤦 Pushing commit to fix that 🤞

[nalimilan]

Thanks!