docs: fix relative paths in docs

bin/processMutationStandardEmoji.js

@@ -33,7 +33,7 @@ async function main () {
       customEmojis.push({
         name,
         shortcodes: [name],
-        url: `/custom/${basename}`,
+        url: `./custom/${basename}`,
         category: categoryName
       })
       await copyFile(file, `./docs/custom/${basename}`)

docs/index.html

@@ -171,7 +171,7 @@
     })
     $('#custom').addEventListener('change', async e => {
       if (e.target.checked) {
-        picker.customEmoji = await (await fetch('/custom.json')).json()
+        picker.customEmoji = await (await fetch('./custom.json')).json()
       } else {
         picker.customEmoji = []
       }