Update plaintext functionality

[cossssmin]

New features

<not-plaintext>

Added a <not-plaintext> tag that you can use to discard content from a plaintext version while keeping it in the HTML.

For example:

<div>Show everywhere</div>
<plaintext>Show in plaintext</plaintext>
<not-plaintext>
  <p>Show only in HTML</p>
</not-plaintext>

Plaintext:

Show everywhere
Show in plaintext

HTML:

<div>Show everywhere</div>
<p>Show only in HTML</p>

Fixes

• fixed an issue with plaintext: true in a template's Front Matter not generating the plaintext file (closes [DOCS] Top level plaintext config does not work in 4.0.0 #707)