upgrade to Documenter 0.25 and DemoCards 0.3

[johnnychen94]

DemoCards v0.3 has brought many new features that I'm eager to upgrade it 😆 It is not a breaking version, the previous build workflow still works, but I'd like to push it forward to the better version.

Author and date meta is added to each demos, which would insert two new badges:

There are two broken jldoctest, I skipped the fragile one and fix the other one in JuliaMath/FixedPointNumbers.jl#229

closes #140
closes #156

[johnnychen94]

The next step is to collect the scattered documentation around JuliaImages: #125 (comment)

The path I have in mind is:

• first wrap the current "packages" part docs with the themeless version of DemoPage as a proxy, and then
• add RemoteDemoCard/RemoteDemoSection/RemoteDemoPage in DemoCards to support docs in remote repo (e.g., ImageFiltering)

It would be something like this:

# Packages
  ## ImageAxes
    introduction.md
  ## ImageFiltering (Remote)
    introduction.md

Whether this goes well is not checked, though.