feat: allow custom kroki host

See https://github.com/gethyas/doks-core/pull/102. Body copied below.

Summary

Allow users to provide a custom Kroki URL instead of hard-coding https://kroki.io.

Basic example

In params.toml, users of this theme can set a custom Kroki URL, such as a local instance:

[doks]
    krokiURL = "http://localhost:8000"

I tried my best to keep proper backwards compatibility should users not provide this configuration key at all—see the diff. Should this not be acceptable, please let me know.

Motivation

At times, the upstream kroki.io service may run into resource exhaustion. This can be due to a high volume of requests or a temporary outage. During an automated build, this isn't so bad—caches should exist and therefore not hit Kroki as often.

However, especially during the writing phase of documentation, where fast iteration is often needed, it is not helpful when the upstream service runs into said resource exhaustion and responds with an unhelpful "Bad Request" error.

This issue is known and understood by the Kroki team; the fix is to run your own instance.^1

Checks

[x] Read Create a Pull Request
[ ] Supports all screen sizes (if relevant)
[ ] Supports both light and dark mode (if relevant)
[ ] Passes npm run test

thuliteio / doks