Improve readme

[TWiStErRob]

Changes

• Restructured hierarchy a bit so nesting is cleaner.
• Revised headers for clarity (minimize wording, remove redundant words, use Detekt terminology)
• Added local rule configuration as the first config, as most of the users will configure via local detekt.yml, not a remote URL.
• Added real example of plugin usage
• Moved "local Detekt plugins" section lower down, as it's more likely people will use plugins from Maven repos.
• Added links to Detekt docs for easy access.

Visual

For testing, preview is available at this PR's branch HEAD.

Before	After

Admin

Feel free to squash these commits on merge.

[TWiStErRob]

cc @dmitry-weirdo thoughts as a user?

[codecov-commenter]

Codecov Report

Merging #170 (bd92f80) into main (9a29854) will not change coverage.
The diff coverage is n/a.

@@            Coverage Diff            @@
##               main     #170   +/-   ##
=========================================
  Coverage     73.93%   73.93%           
  Complexity       37       37           
=========================================
  Files             9        9           
  Lines           165      165           
  Branches         39       39           
=========================================
  Hits            122      122           
  Misses           17       17           
  Partials         26       26           

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