Necessary website changes

[jtojnar]

After #756 is merged, we should do the following:

• Change Sam’s name: Update your website/release notes/docs/etc. #543 (comment)
• Look at the old reference docs and check if there is something useful we should preserve into PHPDoc.
• Drop the old reference docs.
• Clean up the plug-in list (maybe just keep a single page with list of links and brief description)
• Figure out if the IRC channel still exists after the Freenode fiasco. Maybe create a new room at Libera or Matrix.
• Fix wiki syntax in docs/content/wiki/misc/release_notes/simplepie_beta_3.html

[jtojnar]

I have created a Matrix room in lieu of the defunct IRC channel:
https://matrix.to/#/#simplepie:matrix.org