Update CONTRIBUTING.md and New Contributors Guide to reflect that anyone can merge PRs after checks and approvals (#10123)

### Summary
As discussed in [this
PR](#10118), anyone can merge
PRs once approved and all CI checks have passed. `CONTRIBUTING.md` was
outdated, saying only people with write-access could merge.

This change:

* Updates CONTRIBUTING.md to reflect the fact anyone can merge their PR
after required approvals and checks.
* Adds step-by-step instructions on this to the New Contributors Guide.

---------

Co-authored-by: jhels <[email protected]>

Author: jhelsby

Diff for: CONTRIBUTING.md

@@ -409,18 +409,9 @@ for basics.
    - If the reviewers have requests or questions, follow up with them.
    - The goal of the reviewer is to ensure that the code in the `main` branch of
      the repo is consistent, maintainable, and of high quality.
-1. Once the PR has been approved,
-   - If you have the "write permission" in this repo, you can merge it yourself
-     by clicking the "Squash and merge" button once it is green and all CI
-     signals are passing.
-   - If you don't have "write permission" in this repo, the reviewer will take
-     care of the PR. The reviewer may import the PR into Meta's internal system
-     to validate it against internal CI.
-   - If the PR is approved but not merged within 5 business days, please comment
-     on the PR to ask about its status.
-   - Note that if the `main` [CI](#continuous-integration) jobs are broken, we
-     will only merge PRs that fix the broken jobs until all critical jobs are
-     fixed.
+1. Once the PR has been approved, you can merge it yourself
+     by clicking the "Squash and merge" button once it is
+     green and all CI signals are passing.
 
  
 

Diff for: docs/source/_static/img/new-contributor-guide/how_to_merge1.png

@@ -279,6 +279,18 @@ Once you've successfully finished local development, it's time to send out your
 
     ![](./_static/img/new-contributor-guide/pr_approval2.png)
 
-    Once you've received the required approval from a core contributor, you can go ahead and merge your PR. Congratulations — you're now an official ExecuTorch contributor!
+6. Once you've received the required approval from a core contributor, you're very nearly done. We just need to make sure all the CI checks have passed, some of which need approval from a maintainer to start:
 
-Great job making it to the end of our guide — we hope you enjoy contributing. Once again, please check out **[Discord Server](https://discord.com/invite/Dh43CKSAdc)** if you want to say hello, ask any questions, or talk about any and all things ExecuTorch. We look forward to receiving your contributions!
+    ![](./_static/img/new-contributor-guide/how_to_merge1.png)
+
+    Once all checks these have all been approved, ran, and passed, you can go ahead and merge your PR. If there's a grey `Update branch` button instead of a green `Merge pull request` button, click that first:
+
+    ![](./_static/img/new-contributor-guide/how_to_merge2.png)
+
+    After a moment, the branch should update with the latest changes, and you'll see the final green `Merge pull request` button:
+
+    ![](./_static/img/new-contributor-guide/how_to_merge3.png)
+
+    Click it to merge your changes into the main codebase. Congratulations — you're now an official ExecuTorch contributor!
+
+Great job making it to the end of our guide — we hope you enjoy contributing. Once again, please check out our **[Discord Server](https://discord.com/invite/Dh43CKSAdc)** if you want to say hello, ask any questions, or talk about any and all things ExecuTorch. We look forward to receiving your contributions!