Improve TaskRun workspaces documentation
Improve TaskRun workspaces documentation

This commit contains similar changes for the TaskRun section as what was done for the PipelineRun section in https://github.com/tektoncd/pipeline/pull/2521

- We did not provide a full example of a TaskRun definition under "Examples of `TaskRun` definition using `Workspaces`"
  This commit add a full example of a TaskRun definition.

- Examples of how to use different volume sources already exists under "Specifying `VolumeSources` in `Workspaces`",
  add a link to that section instead of having the same examples under the TaskRun section.

- A few broken links in the table-of-contents are fixed.

/kind documentation

tektoncd/pipeline,tektoncd/pipeline