Enhancement: Dynamic title attribute on Markdown Footnote links

### Impacted plugin

Jetpack

### What

Footnote links' titles say "Read footnote." This would make the attribute display the content of the footnote.

### How

At the moment, if a user hovers over a footnote, the `title` attribute displays this tool tip:

---

I think it would be nice if JetPack showed users the *full* text of the footnote. For example:

---

This is controlled by :

https://github.com/Automattic/jetpack/blob/d7a656966df2ca3c78a3ac4b66b0bf0ea2e4047d/projects/plugins/jetpack/_inc/lib/markdown/extra.php#L3020

Specifically:

```php
if ($this->fn_link_title != "") {
$title = $this->fn_link_title;
$title = $this->encodeAttribute($title);
$attr .= " title=\"$title\"";
}
```

The change is relatively straightforward:

```php
if ($this->fn_link_title != "") {
$title = trim( wp_strip_all_tags($this->footnotes[$node_id], true) );
$title = $this->encodeAttribute($title);
$attr .= " title=\"$title\"";
}
```

That takes the text of the footnote, sanitises it, and adds it as the title element.

This builds on the work I did in #21371

Start in projects/plugins/jetpack/_inc/lib/markdown/extra.php around the fn_link_title handling at line 3020, and inspect how the footnote text is stored in $this->footnotes. Done means the generated footnote link title contains the trimmed, sanitized footnote content instead of the fixed “Read footnote” text.

Written by the indexing model from the issue text.