feat(analytics): Goatcounter support (#956)

* Add options to support goatcounter analytics

* goatcounter: support self-hosted

* Add to configuration docs for goatcounter settings

* use https instead of protocol-relative link for goatcounter js

docs/configuration.md

@@ -28,6 +28,7 @@ This part of the configuration concerns anything that can affect the whole site.
   - `{ provider: 'google', tagId: '<your-google-tag>' }`: use Google Analytics;
   - `{ provider: 'plausible' }` (managed) or `{ provider: 'plausible', host: '<your-plausible-host>' }` (self-hosted): use [Plausible](https://plausible.io/);
   - `{ provider: 'umami', host: '<your-umami-host>', websiteId: '<your-umami-website-id>' }`: use [Umami](https://umami.is/);
+  - `{ provider: 'goatcounter', websiteId: 'my-goatcounter-id' }` (managed) or `{ provider: 'goatcounter', websiteId: 'my-goatcounter-id', host: 'my-goatcounter-domain.com', scriptSrc: 'https://my-url.to/counter.js' }` (self-hosted) use [GoatCounter](https://goatcounter.com)
 - `locale`: used for [[i18n]] and date formatting
 - `baseUrl`: this is used for sitemaps and RSS feeds that require an absolute URL to know where the canonical 'home' of your site lives. This is normally the deployed URL of your site (e.g. `quartz.jzhao.xyz` for this site). Do not include the protocol (i.e. `https://`) or any leading or trailing slashes.
   - This should also include the subpath if you are [[hosting]] on GitHub pages without a custom domain. For example, if my repository is `jackyzha0/quartz`, GitHub pages would deploy to `https://jackyzha0.github.io/quartz` and the `baseUrl` would be `jackyzha0.github.io/quartz`.