update docs + add api doc autogenerate funcs to Makefile

[yokawasa]

related issue https://github.com/st-tech/gatling-operator/issues/22

Description

• add docs generating function to Makefile: https://github.com/st-tech/gatling-operator/commit/15210bfccc345f561c3ab2d894cb0d1a5506622e
  • Now that API references can be generated with make docs
• add crd-ref-docs configuration files which are used to generate API references doc from CRD types https://github.com/st-tech/gatling-operator/commit/d559063a635a2493803c109e40587469a51f705d
• Updated docs
  • updated README.md (added architecture image, overview explanation, features, etc.)
  • add API references
  • add developer guides

Test

testing on make docs

make docs

As expected api.md file was generated from CRD types under docs directory

[yokawasa]

@tmrekk121 @akitok @kayamin @takuya-mikami thanks for the review! let me go ahead to merge the PR