docs: Use proper subheadings for all sections in UI documentation

Signed-off-by: Harri Lehtola <peruukki@hotmail.com>

docs/reference/alpha-web-ui.md

@@ -100,7 +100,7 @@ yarn start
 
 The advantage of importing Feast UI as a module is in the ease of customization. The `<FeastUI>` component exposes a `feastUIConfigs` prop thorough which you can customize the UI. Currently it supports a few parameters.
 
-**Fetching the Project List**
+##### Fetching the Project List
 
 You can use `projectListPromise` to provide a promise that overrides where the Feast UI fetches the project list from.
 
@@ -118,7 +118,7 @@ You can use `projectListPromise` to provide a promise that overrides where the F
 />
 ```
 
-**Custom Tabs**
+##### Custom Tabs
 
 You can add custom tabs for any of the core Feast objects through the `tabsRegistry`.
 
@@ -142,7 +142,7 @@ const tabsRegistry = {
 
 Examples of custom tabs can be found in the `ui/custom-tabs` folder.
 
-**Custom basename**
+##### Custom basename
 
 If you want to serve the UI under a specific URL path instead of at the root,
 you can use the `basename` prop, and it will be forwarded to React Router.