Add developer docs and a note about abandonment

[timholy]

I hereby retire as maintainer of this package. I have written too many packages to support them all, and since this package has a wide user base it should not be too difficult to find someone to fill my shoes.

To facilitate the transition, this PR adds a whole bunch of developer-oriented documentation. It also posts a note on the README advertising for a new maintainer.

[timholy]

Preview the documentation from this PR: https://juliamath.github.io/Interpolations.jl/previews/PR365/. In particular click on the "Developer documentation." But there are also many new docstrings for internal (and even exported) objects.

[timholy]

xref https://discourse.julialang.org/t/interpolations-jl-needs-a-new-maintainer/37454

[tomasaschan]

It's been a looong time since I did anything in the Julia community, but when I started out - by reaching out to you, @timholy, about adding cubic spline interpolations to Grid.jl - I could not have been given a warmer welcome. You took me under your wings for a while, coached me into building the first versions of this package (I assume all of that code is more or less replaced by now) and taking care of it for a few years, and you took on a large part of the burden when my career path took me elsewhere and I didn't find time to do my part as a maintainer anymore (see #140, ages ago).

Thank you.

I just wanted to ensure that got said (with enough emphasis).

[timholy]

Thanks @tomasaschan! I've always enjoyed collaborating with you on this package. Cheers to its bright future in new hands!