Restructure generated site data for searchbar ui

[ang-zeyu]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [x] Enhancement to an existing feature

Resolves #994
Requires MarkBind/vue-strap#126 ( searchbar ui for vue-strap )

What is the rationale for this request?
Generated site data for the searchbar needs to be restructured to sift out the keywords from the heading deterministically.

What changes did you make? (Give an overview)
Generated site data:

• headings now store only the id:'heading text...' pair instead of id:'heading text... | concatenated keywords ...'
• keywords for headings are now stored in the headingKeywords field
• Removed concatenateHeadingsAndKeywords which is no longer needed

Provide some example code that this change will affect:

New site data:

const siteData = {
    enableSearch: this.siteConfig.enableSearch,
    pages: this.pages.filter(page => page.searchable)
      .map(page => ({
        ...page.frontMatter,
        headings: page.headings,
        headingKeywords: page.keywords,
      })),
  };

Is there anything you'd like reviewers to focus on?
na

Testing instructions:
npm run test should pass

Proposed commit message: (wrap lines at 72 characters)

Restructure generated site data for searchbar ui

Keywords tied to a heading are appended to the heading with a prepending
'|' character.
This is so that the '|' character would serve as a visual separator
between the heading and the keyword.

However, it is thus not possible to extract a keyword from its heading
deterministically, as a heading can also include the '|' character.

Let's move the keywords to its own field, allowing the searchbar to
extract a heading's keyword from it.

[ang-zeyu]

Rebased on latest, no conflicts

[ang-zeyu]

Rebased on the latest master, no conflicts / changes otherwise