Remove solid background blocks from documentation series titles in docs_changelog.html

**Your Godot version:**
N/A

**Issue description:**
In the Documentation changelog page, the text lines introducing manual document series ("Procedural geometry series:" and "Your First Shader Series:") are displayed in a solid background that mismatches with the rest of the page background (gray in light mode and black in dark mode).

While this is likely *by design*, it breaks the visual harmony and clean layout of the rest of the documentation theme.

**URL to the documentation page:**
https://docs.godotengine.org/en/latest/about/docs_changelog.html

**Attachments:**
Light mode:

Dark mode:

Open docs_changelog.html and inspect the two manual document series titles in both light and dark modes, then trace their documentation styling. Done means the titles no longer display solid background blocks and visually match the surrounding page background in both themes.

Written by the indexing model from the issue text.