feat: Styling for blockquotes

[mgax]

Fix https://github.com/ietf-tools/www/issues/321.

Here is what the blockquotes look like with this patch:

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 82.72%. Comparing base (4ad3143) to head (b2e7e64). Report is 7 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #402 +/- ## ======================================= Coverage 82.72% 82.72% ======================================= Files 124 124 Lines 2599 2599 ======================================= Hits 2150 2150 Misses 449 449 ```

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[mgax]

Looking good, just need to tweak the text colour for accessibility.

Good point, and thanks for the resource! The text colour is more similar to the body text, but it's slightly heavier, and there's the indentation bar, so I don't think that will be a problem.