docs/developer links to wrong pages on Firefox/Safari

Nobody has claimed this yet.

**Describe the bug**
On Firefox and Safari the [matrix.org/docs/develop](https://matrix.org/docs/develop/) page looks different than on chromoium and link title don't match the actual link behind it.

**To Reproduce**
Steps to reproduce the behavior:
1. Open [matrix.org/docs/develop](https://matrix.org/docs/develop/) using Firefox or Safari
2. Insect a Link towards the bottom of the page (Use Inspect tool or hover over link and see link preview)
4. Link doesn't lead where the text suggest it does (for example "Types of Bridging" leads to "Introduction")

**Expected behavior**
Link text should match link

**Screenshots**

**Desktop (please complete the following information):**
- OS: Ubuntu 20.10 (but also occurs on other systems, for example Android)
- Browser: Firefox (but according to @aaron:raim.ist it also occurs on Safari)
- Version: 86.0

**Smartphone (please complete the following information):**
- Device: Pixel 4a 5G
- OS: Android 11
- Browser: Firefox
- Version: 86.1.1

**Additional context**
This issue does **not** exist on Chrome/Brave

Start with the matrix.org/docs/develop/ page and reproduce the mismatched link text and targets in Firefox or Safari, then compare the same links in Chrome/Brave. Done means the visible titles and destinations agree across the reported browsers, including the “Types of Bridging” example.

Written by the indexing model from the issue text.