[doc] add documentation index file

[inkydragon]

fix #81

Should we split out the API usage section in the README and put it in the docs?

Then leave a link to the docs in the README.

It seems that the introduction of the functions are directly copied from the docstring.

Then we just need to move the "Compression" and "API comparison" sections to docs.
We can also move "Design & Features" to docs if necessary.

[codecov]

Codecov Report

Merging #124 (a9fd7f4) into master (67f004d) will decrease coverage by 0.32%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #124      +/-   ##
==========================================
- Coverage   96.84%   96.52%   -0.33%     
==========================================
  Files           4        4              
  Lines         729      776      +47     
==========================================
+ Hits          706      749      +43     
- Misses         23       27       +4     

Impacted Files	Coverage Δ
src/Tar.jl	90.80% <0.00%> (-1.97%)	⬇️
src/header.jl	93.33% <0.00%> (-1.12%)	⬇️
src/create.jl	96.84% <0.00%> (-0.35%)	⬇️
src/extract.jl	98.11% <0.00%> (+0.13%)	⬆️

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[StefanKarpinski]

I'd say all of that would be an improvement—anything you think makes this clearer I'm perfectly happy with. I've been putting the docs in the READMEs for this little self-contained packages because it seems easier than having a separately hosted doc site, but since these are stdlibs, there's already a natural place for those docs to go in the corresponding Julia manual section, so putting the docs there would make sense.