add "theme" key to page's config file and deprecate direct usage of cardtheme

[johnnychen94]

Two improvements:

• unify makedemos API
• simplify preview_demos usage

unify makedemos API

64 introduces a new way of how demos are navigated, which I call "themeless". The downside of it is that makedemos becomes more complex to configure, this PR unifies how themeless and themed versions are used, and make it even simpler.

Previous:

# themed version

demos_theme, demos_assets = cardtheme("grid")
demos, demos_cb = makedemos("examples", demos_theme)
makedocs(format = Documenter.HTML(assets=[demos_assets]),
    pages = [
        "Home" => "index.md",
        demos
    ]
)
demos_cb()

# themeless version
demos, demos_cb = makedemos("examples")
makedocs(format = Documenter.HTML(),
    pages = [
        "Home" => "index.md",
        demos
    ]
)
demos_cb()

Now:

# works for both themed and themeless verions

demos, demos_cb = makedemos("examples")

isnothing(demo_assets) || (push!(assets, demo_assets))
format = Documenter.HTML(assets=assets)

makedocs(format = format,
    pages = [
        "Home" => "index.md",
        demos
    ]
)

demos_cb()

The page theme is configured via config.json file, for example:

{
    "theme": "grid",
    "order": [
        "basic",
        "advanced"
    ]
}

Currently, "theme" can be one of "grid", "list" or "nothing", where "nothing" represents the themeless case. If you don't configure "theme", it would defaults to nothing.

Simplify preview_demos usage

Previously, we have to manually write preview_demos("examples", theme="grid") to preview a grid-like demos.

Now, it will infer the theme from config.json, and if that's already configured grid, preview_demos("examples") would just work out of the box.

preview_demos("examples", theme="list") would instead override the "grid" theme in "config.json" -- I made it so because if the user explicitly passes a keyword to theme, I believe he knows what he's expecting.

---

It's a bit hard to add deprecation tests for this, I've tested it locally that the old way still works. I'd save myself a bit of time and skip that step. 😂 The coverage regression isn't problematic.

[codecov[bot]]

Codecov Report

Merging #66 into master will decrease coverage by 0.47%. The diff coverage is 80.76%.

@@            Coverage Diff             @@
##           master      #66      +/-   ##
==========================================
- Coverage   91.74%   91.27%   -0.48%     
==========================================
  Files          14       14              
  Lines         654      676      +22     
==========================================
+ Hits          600      617      +17     
- Misses         54       59       +5     

Impacted Files	Coverage Δ
src/generate.jl	81.66% <60.00%> (-1.38%)	:arrow_down:
src/types/page.jl	96.25% <90.90%> (-0.98%)	:arrow_down:
src/preview.jl	93.54% <100.00%> (+0.36%)	:arrow_up:

---

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