Introduce a feature to skip a single page TOC validation

[mino181295]

In this pull request I introduced a meta tag

<meta name="docet-hidden-page" >

To skip the TOC unlinked file validation.
Writing the head tag with inside this meta you can still pass the validation of the documentation page.

[eolivelli]

Awesome.
But we are missing one step: while serving the HTML to the client we have to remove the META tag.

We could also add a minimal test case about the serve side part (a unit test about the removal of the tag)

[mino181295]

It's built in the DocetManager#servePageIdForLanguageForPackage that when you serve a html/pdf page you never include what is inside the head of the document and some tags are cleaned (like <script> tags)

case TYPE_HTML:
     html.append(htmlDoc.body().getElementsByTag("div").first().html());
     html.append(generateFooter(lang, packageName, pageId, faq));
     res = DocetUtils.cleanPageText(html.toString());

For the test: to follow all the DocetManager#serveRequest it's a long run to mock/follow all the page request and discorvery.