docs(examples): add UnixFS file download over Bitswap

[aschmahmann]

This is a starting point to discuss how we want to manage our examples for go-libipfs. It also IMO gives some illustration on some of the UX pains of working with the existing libraries, places where we can add nicer Go helper utilities, etc.

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[codecov]

Codecov Report

Merging #143 (3f756da) into main (c394290) will increase coverage by 0.32%.
The diff coverage is 41.21%.

@@            Coverage Diff             @@
##             main     #143      +/-   ##
==========================================
+ Coverage   29.19%   29.52%   +0.32%     
==========================================
  Files          96      100       +4     
  Lines       11018    11237     +219     
==========================================
+ Hits         3217     3318     +101     
- Misses       7448     7548     +100     
- Partials      353      371      +18     

Impacted Files	Coverage Δ
examples/unixfs-file-cid/main.go	41.21% <41.21%> (ø)
files/filewriter_windows.go	100.00% <0.00%> (ø)
files/is_hidden_windows.go	82.35% <0.00%> (ø)
tar/sanitize_windows.go	41.66% <0.00%> (ø)

[hacdias]

I'm happy to merge this PR once the remanning dependency conflicts are solved. I agree that there is a lot of complexity and I think that is something we should work on by creating helper functions for common use-cases.