Add info message on empty toc (#567)

public/locales/en.json

@@ -153,6 +153,7 @@
         "untitledNote": "Untitled",
         "placeholder": "← Start by entering a title here\n===\nVisit the features page if you don't know what to do.\nHappy hacking :)",
         "invalidYaml": "The yaml-header is invalid. See <0></0> for more information.",
+        "infoToc": "Structure your note with headings to see a table-of-contents here.",
         "help": {
             "shortcuts": {
                 "title": "Shortcuts",

src/components/editor/table-of-contents/table-of-contents.tsx

@@ -1,4 +1,5 @@
 import React, { Fragment, ReactElement, useMemo } from 'react'
+import { Trans, useTranslation } from 'react-i18next'
 import { TocAst } from '../../../external-types/markdown-it-toc-done-right/interface'
 import { ShowIf } from '../../common/show-if/show-if'
 import './table-of-contents.scss'
@@ -52,10 +53,14 @@ const convertLevel = (toc: TocAst, levelsToShowUnderThis: number, headerCounts:
 }
 
 export const TableOfContents: React.FC<TableOfContentsProps> = ({ ast, maxDepth = 3, className }) => {
+  useTranslation()
   const tocTree = useMemo(() => convertLevel(ast, maxDepth, new Map<string, number>(), false), [ast, maxDepth])
 
   return (
     <div className={`markdown-toc ${className ?? ''}`}>
+      <ShowIf condition={ast.c.length === 0}>
+        <Trans i18nKey={'editor.infoToc'}/>
+      </ShowIf>
       { tocTree }
     </div>
   )