Clarify language about GH contributions being accepted

For instructor training checkout, we note one way to "Get Involved" is to [Contribute to the construction or maintenance of Carpentries materials](https://carpentries.github.io/instructor-training/checkout.html#contribute-to-the-construction-or-maintenance-of-carpentries-materials). In that section, we note "Your contribution does not need to be reviewed or accepted in order to satisfy your ‘Get Involved’ step."

In the section about submitting your contribution in AMY, we note "Your contribution should be evaluated in 7-10 days. We suggest checking back to ensure that your contribution was accepted."

We often get asked if a GH issue/PR must be accepted for it to count towards the "Get involved" step. We use "accepted" in two senses above - one referring directly to the GH issue/PR and one referring to submitting a record of it in AMY. This can be confusing and we get questions about whether a GH contribution must be accepted in order to count. Consider re-phrasing some of this language. Perhaps something like "Your submission should be evaluated in 7-10 days. If your submission remains pending after this time, please contact instructor.training@carpentries.org."

Start with the instructor training checkout page at the linked “Get Involved” section and the section describing submission in AMY. Compare the two uses of “accepted,” then revise the wording so it clearly distinguishes acceptance of a GitHub contribution from evaluation of its AMY submission. Done means the page no longer suggests that a GitHub issue or pull request must be accepted to count.

Written by the indexing model from the issue text.