chore(github): comment out notational pieces of PR template (#1431)

- functionally identical to an old PR of mine in a different repo: argoproj/argo-helm@429b474
- the notes about versioned docs are purely notational, i.e. have nothing to fill out
  - comment them out with HTML comments, as is common practice
    - example from a repo I maintain: https://github.com/ezolenko/rollup-plugin-typescript2/blob/f6db59613a66f58c48310aa8fa785951970b5d6d/.github/issue_template.md?plain=1#L2
      - I copied that from other repos too
  - these comments are still visible to the PR author, just not visible when rendered, keeping the PR more concise

Author: agilgur5

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,3 +1,5 @@
+<!--
+
 # READ ME PLEASE
 
 > **TL;DR: Make sure to add your changes to versioned docs**
@@ -7,3 +9,5 @@ Thanks for opening a PR!
 The docs cover several versions of `react-navigation`, and in some cases there are several files (for version 1, version 2 and etc.) that all describe a single page of the docs (eg. "Getting Started").
 
 Please make sure that the edit you're making in `docs/file-you-edited.md` is also included in the file for the correct version, eg. `/versioned_docs/version-3.x/file-you-edited.md` for version 3. If such file doesn't exist, please create it. :+1:
+
+-->