Revamp Tekton developer docs for /tekton paths.

The content is largely the same, this just reorganizes some of the
content into a table and provides a sample filetree to show what
files/directories are present.

Additionally, this adds docs for the following directories that were not
already present:

  - /tekton/downward
  - /tekton/creds

Other diffs are formating caused by `prettier --prose-wrap always`