Broken links to contribution guidelines

[bootstrapbill]

Proposed documentation enhancement

These two links are dead:

https://github.com/mne-tools/mne-python/blob/ae6e55e8eb41f080d6c209c129bb86ecdb2fc30d/.github/CONTRIBUTING.md?plain=1#L13C1-L13C127

https://github.com/mne-tools/mne-python/blob/ae6e55e8eb41f080d6c209c129bb86ecdb2fc30d/.github/PULL_REQUEST_TEMPLATE.md?plain=1#L1C1-L3C19

Looks like they were just missed in #12316.

[welcome]

Hello! 👋 Thanks for opening your first issue here! ❤️ We will try to get back to you soon. 🚴