Update issue and pull request templates

.github/ISSUE_TEMPLATE/BUG_REPORT.md

@@ -7,6 +7,10 @@ about: Create a report to help us improve
 <!--
 All bug reports must provide the following background information
 Text between <!-- and --​> marks will be invisible in the report.
+
+IF YOUR ISSUE IS CONSIDERED A SECURITY VULNERABILITY THEN PLEASE STOP AND
+DO NOT POST IT AS A GITHUB ISSUE! Please report the issue responsibly by
+disclosing in private by email to security@matrix.org instead.
 -->
 
 ### Background information
@@ -18,7 +22,6 @@ Text between <!-- and --​> marks will be invisible in the report.
 - **`go version`**:
 - **Client used (if applicable)**:
 
-
 ### Description
 
 - **What** is the problem:
@@ -38,7 +41,6 @@ Examples of good descriptions:
 - How: "Lots of logs about device change updates"
 - When: "After my server joined Matrix HQ"
 
-
 Examples of bad descriptions:
 - What: "Can't send messages"  - This is bad because it isn't specfic enough. Which endpoint isn't working and what is the response code? Does the message send but encryption fail?
 - Who: "Me" - Who are you? Running the server or a user on a Dendrite server?

.github/PULL_REQUEST_TEMPLATE.md

@@ -1,8 +1,8 @@
 ### Pull Request Checklist
 
-<!-- Please read docs/CONTRIBUTING.md before submitting your pull request -->
+<!-- Please read https://matrix-org.github.io/dendrite/development/contributing before submitting your pull request -->
 
 * [ ] I have added tests for PR _or_ I have justified why this PR doesn't need tests.
-* [ ] Pull request includes a [sign off](https://github.com/matrix-org/dendrite/blob/main/docs/CONTRIBUTING.md#sign-off)
+* [ ] Pull request includes a [sign off below using a legally identifiable name](https://matrix-org.github.io/dendrite/development/contributing#sign-off) _or_ I have already signed off privately
 
 Signed-off-by: `Your Name <your@email.example.org>`