Feature: support templating in redoc.html

[dhruvkb]

Describe the feature
The redoc.html file included in the project templates (https://github.com/tfranzel/drf-spectacular/blob/28b858e83e8341c806cc23e7cb09ea1f06b2b357/drf_spectacular/templates/drf_spectacular/redoc.html) is great as a Redoc site starting point, but a mature project needs to also include some custom code.

Expected behavior
Changing the HTML template to include some blocks would project a lot of benefits such as the ability to

• include some custom CSS
  • possibly implement a dark theme
• add a favicon to the site
• add open-graph meta tags

Currently there is no known or documented way to achieve these goals.

[tfranzel]

We added a lot of customization for swagger-ui and it was never really enough. I would prefer to not add 3-4 more settings just for redoc.

We finally added blocks in #918, which is flexible and not that settings-heavy. I could image having the same for redoc.

In any case, you can always override a template (via app ordering) for any django app yourself. Nothing special going on here. The blocks version is just a less copy&pasty version of that.

[dhruvkb]

Yes, my requests was precisely to support blocks in the Redoc template. Happy to make a PR that adds {% block %}s if you would accept.

As a user, we could overwrite the entire template, but that would mean losing out on any improvements brought by future updates to drf-spectacular unless we manually check and keep comparing the two versions.