Fix: update contracts folder structure

[VGau]

This PR implements issue(s) #399

Checklist

• I wrote new tests for my new core changes.
• I have successfully ran tests, style checker and build against my new changes locally.
• I have informed the team of any breaking changes if there are any.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 60.54%. Comparing base (4a7b76c) to head (94ba201).

Additional details and impacted files

@@             Coverage Diff              @@
##               main     #420      +/-   ##
============================================
- Coverage     68.22%   60.54%   -7.68%     
+ Complexity     1128     1061      -67     
============================================
  Files           319      348      +29     
  Lines         12795    14953    +2158     
  Branches       1277     1697     +420     
============================================
+ Hits           8729     9054     +325     
- Misses         3540     5427    +1887     
+ Partials        526      472      -54     

Flag	Coverage Δ		*Carryforward flag
hardhat	37.52% <ø> (-61.19%)	⬇️
kotlin	68.06% <ø> (+2.21%)	⬆️	Carriedforward from d6c21da

*This pull request uses carry forward flags. Click here to find out more.

Files with missing lines	Coverage Δ
...esting/integration/IntegrationTestTrueVerifier.sol	0.00% <ø> (ø)
...sting/integration/L2MessageServiceLineaMainnet.sol	12.19% <ø> (ø)
...ntracts/src/_testing/integration/LineaRollupV5.sol	9.97% <ø> (ø)
contracts/src/_testing/integration/ProxyAdmin.sol	0.00% <ø> (ø)
...ts/src/_testing/integration/TokenBridgeFlatten.sol	9.25% <ø> (ø)
...esting/integration/TransparentUpgradeableProxy.sol	0.00% <ø> (ø)
...acts/src/_testing/mocks/base/RevertingVerifier.sol	88.88% <ø> (ø)
...cts/src/_testing/mocks/base/TestClaimingCaller.sol	100.00% <ø> (ø)
...s/src/_testing/mocks/base/TestL1RevertContract.sol	0.00% <ø> (ø)
.../src/_testing/mocks/base/TestReceivingContract.sol	0.00% <ø> (ø)
... and 67 more

... and 51 files with indirect coverage changes

[kyzooghost]

Can I suggest that we also update the autogenerated documentation in contracts/docs/api?