move ta-generator to task-generator directory

[tnevrlka]

ta-generator and task-generator are currently on the same directory level.
Move ta-generator so it's a subdirectory of the task-generator leading to a better file organization in the repository.

• update .gitignore so it ignores vendor directories and diff
  files used in trusted-artifacts task generation testing
• reflect the move of the trusted-artifacts
  task generator to the task-generator directory
  in the README
• add a README into every task-generator subdirectory

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[chmeliik]

Optionally, we could make the task-generator directory a Go workspace to tie the two submodules together.

But TBH, I'm not entirely sure what problems Go workspaces solve, so we can leave it as is

[tnevrlka]

Thank you for the review!

I would like to avoid using Go workspace if possible. The Go Modules reference mentions It is generally inadvisable to commit go.work files into version control systems and I'm not familiar with Go workspaces enough to understand if our use case warrants it.