Add blocks to base.html and index.html to make them easier to extend

[helenb]

Description

Add blocks to base.html and index.html to make them easier to extend

Fixes # (issue)

#276

Checklist

• My code follows the style guidelines of 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
• My changes generate no new warnings
• 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
• I have added an appropriate CHANGELOG entry

[thibaudcolas]

ty! please don’t edit the CHANGELOG in PRs as it creates lots of conflicts when merging