Update /en/wiki/verify-trisquel-download

I updated https://trisquel.info/en/wiki/verify-trisquel-download as a follow up from discussion https://trisquel.info/en/forum/gpg-key-doesnt-match-what-website-says where Ark74 had already written updated instructions (as I was seeking to verify an .iso file I downloaded).

I added the instructions Ark74 had written; I have a few comments:

1. the now former wiki page included information about how to verify the SHA256 hash. I left this how-to as part 4, after the 3 parts Ark74 wrote.

2. the now former wiki page, last updated in 2014, stipulated that md5 is deprecated . I did not know whether instructions should be kept or deleted. I left them.

3. I don't know if the following (untouched at the bottom of the page) is still relevant:

> An alternative method is to download the trisquel-archive-signkey.gpg file and run: gpg --import trisquel-archive-signkey.gpg
> This will allow you to import the key without having to connect to a keyserver. Useful for Tor users.

> Tip: If downloading from torrent via Transmission-GTK you may use "Verify Local Data" to correct any corrupt data from a download.

I left it.

4. The updated instructions from Ark74 also include the download of the iso and the key. At the moment, the title of the page is "Verify Trisquel Download", maybe the page should be renamed "Download and Verify Trisquel files" or some name to reflect that the wiki page provides a how-to-download-and-verify. There is also this other page https://trisquel.info/en/wiki/download-trisquel titled "download" but including (duplicated?) information about "verifying Trisquel".