text: Title: Documentation - Update page titles to align better to writing style guide
  Body: Our writing style guide advises we should align with the Google developer documentation style guide. However, some page titles are still using an inconsistent style.
  
  * https://docs.wagtail.org/en/latest/contributing/documentation_guidelines.html#writing-style-guide
  * https://developers.google.com/style/headings
  
  ### Pertinent section of the Wagtail docs
  
  There are a few pages that have inconsistent main titles that do not adhere to the usage of sentence case.
  
  > Use sentence case for headings and titles.
  
  These are minor changes but help us present a consistent tone in our documentation (at least in the TOC - Table of Contents).
  
  ### Details
  
  These are the ones I have found, there could be others.
  
  | URL | Change to make |
  |-|-|
  | https://docs.wagtail.org/en/latest/advanced_topics/images/feature_detection.html | `Feature detection` (lower case d) |
  | https://docs.wagtail.org/en/latest/advanced_topics/api/v2/configuration.html | `Wagtail API v2 configuration guide` (lower case c & g) |
  | https://docs.wagtail.org/en/latest/advanced_topics/api/v2/usage.html | `Wagtail API v2 usage guide` (lower case u & g) |
  
  We do have the page [Managing the Reference Index](https://docs.wagtail.org/en/latest/advanced_topics/reference_index.html), we either need to change the title to be lower case 'reference index' or update the content to consistently use the proper noun 'Reference Index'.
  
  
  ### Working on this
  
  Anyone can contribute to this. View our [contributing guidelines](https://docs.wagtail.org/en/latest/contributing/index.html), add a comment to the issue once you’re ready to start.
  