Add file extension to links

[eike-hass]

Description of change

Add file extension to (hopefully) all links to fix issues with links not being resolved as can be seen in the chapter links here.

Links to any relevant issues

Be sure to reference any related issues by adding fixes issue #.

Type of change

Add an x to the boxes that are relevant to your changes.

• Bug fix (a non-breaking change which fixes an issue)
• Enhancement (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Documentation Fix

How the change has been tested

Tested by @Dr-Electron here.

Change checklist

Add an x to the boxes that are relevant to your changes.

• I have followed the contribution guidelines for this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[Dr-Electron]

Seems to have fixed it. You can check it here:
https://iota-wiki-it-iota-iota1.vercel.app/shimmer/identity.rs/introduction/

[eike-hass]

Blocked by an unrelated error:
error[E0599]: no method named with_burning_allowedfound for structClientBlockBuilder in the current scope

[eike-hass]

Merging despite the unrelated build failure to get the changes in the Wiki ASAP.