user-guide.md: add a few examples and remove mentions about futures

[kingslef]

Add a few examples to the user guide docs and remove the mentions about futures so that it is a bit closer to what's actually in finchx.

Also add a link to giter8 template to the index.

[codecov-commenter]

Codecov Report

Merging #1268 into master will decrease coverage by 1.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1268      +/-   ##
==========================================
- Coverage   81.16%   80.07%   -1.10%     
==========================================
  Files          55       55              
  Lines        1009     1009              
  Branches       42       38       -4     
==========================================
- Hits          819      808      -11     
- Misses        190      201      +11     

Impacted Files	Coverage Δ
core/src/main/scala/io/finch/DecodeEntity.scala	78.26% <0.00%> (-21.74%)	⬇️
core/src/main/scala/io/finch/DecodePath.scala	90.00% <0.00%> (-10.00%)	⬇️
fs2/src/main/scala/io/finch/fs2/package.scala	52.17% <0.00%> (-7.09%)	⬇️
...tee/src/main/scala/io/finch/iteratee/package.scala	71.42% <0.00%> (-3.58%)	⬇️
core/src/main/scala/io/finch/Endpoint.scala	71.03% <0.00%> (-1.74%)	⬇️
...e/src/main/scala/io/finch/endpoint/multipart.scala	78.84% <0.00%> (-1.55%)	⬇️
core/src/main/scala/io/finch/Error.scala	55.55% <0.00%> (ø)
core/src/main/scala/io/finch/Input.scala	100.00% <0.00%> (ø)
core/src/main/scala/io/finch/Trace.scala	78.57% <0.00%> (ø)
core/src/main/scala/io/finch/Accept.scala	81.81% <0.00%> (ø)
... and 29 more

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