Update user-facing documentation for edit button

docs/customisation/edit-button.md

@@ -2,7 +2,9 @@
 
 Furo can add a small edit button to each document.
 
-This is automatically added, when the documentation is generated on Read the Docs using a GitHub repository as the source. It is possible to provide this, outside of Read the Docs, by setting the following keys in [`html_theme_options`][sphinx-html_theme_options]:
+## With popular VCS hosts
+
+Provide the relevant VCS variables, by setting the following keys in [`html_theme_options`][sphinx-html_theme_options]:
 
 ```python
 html_theme_options = {
@@ -12,8 +14,34 @@ html_theme_options = {
 }
 ```
 
-## Disabling on Read the Docs
+This model supports github.com, gitlab.com and bitbucket.org as domain names for the `source_repository` key.
+
+## With arbitrary URLs
+
+```{versionadded} 2022.09.29
+
+```
+
+Use arbitrary URLs for the edit button, by setting the following key in [`html_theme_options`][sphinx-html_theme_options]:
+
+```python
+html_theme_options = {
+    "source_edit_link": "https://my.awesome.host.example.com/awesome/project/edit/{filename}",
+}
+```
+
+The `{filename}` component will be replaced with the full path to the file, as known from the base of the documentation directory.
+
+```{important}
+Furo does not enforce that the `source_edit_link` contains `{filename}` or any sort of correctness check on this URL. Make sure to manually confirm that the link works.
+```
+
+## Read the Docs support
+
+If you're building documentation on Read the Docs using a github.com-hosted repository as the source, the edit button is enabled by default.
+
+### Disabling on Read the Docs
 
-If you're building documentation on Read the Docs using a GitHub repository as the source, the edit button is enabled by default. If you wish to disable this, use {ref}`top_of_page_button`.
+If you wish to disable this, use {ref}`top_of_page_button` and set it to `None`.
 
 [sphinx-html_theme_options]: https://www.sphinx-doc.org/en/master/usage/configuration.html#confval-html_theme_options