Resolve "The URLs in the documentation linking to the Kraken API documentation are out-dated"

[btschwertfeger]

Summary

Updating all those URLs of the documentation.

Closes #358

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 85.94%. Comparing base (57647c0) to head (a8995c8).
Report is 11 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #360      +/-   ##
==========================================
- Coverage   93.58%   85.94%   -7.65%     
==========================================
  Files          25       25              
  Lines        1777     2113     +336     
==========================================
+ Hits         1663     1816     +153     
- Misses        114      297     +183     

Flag	Coverage Δ
unittests	85.94% <ø> (-7.65%)	⬇️

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Files with missing lines	Coverage Δ
src/kraken/exceptions/__init__.py	99.34% <ø> (ø)
src/kraken/futures/funding.py	66.66% <ø> (ø)
src/kraken/futures/market.py	91.57% <ø> (ø)
src/kraken/futures/trade.py	94.56% <ø> (ø)
src/kraken/futures/user.py	86.15% <ø> (ø)
src/kraken/futures/ws_client.py	89.62% <ø> (ø)
src/kraken/spot/earn.py	32.50% <ø> (ø)
src/kraken/spot/funding.py	45.83% <ø> (ø)
src/kraken/spot/market.py	100.00% <ø> (ø)
src/kraken/spot/orderbook.py	94.93% <ø> (ø)
... and 4 more

... and 36 files with indirect coverage changes

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