Community how-to: explicit private-email and pseudonymous-posting instructions

[eaitbrahim]

Extends the #69 how-to-post section (from #48): users asked how to create a GitHub account, keep their email private, and post "anonymously" in keel's discussions.

Add to the community page how-to (en/ar/fr), staying honest per #48's constraint (never claim true anonymity — GitHub sees the account; readers see only the username):

• A step or note: after signup, make the email private — GitHub → Settings → Emails → enable "Keep my email addresses private" (GitHub then shows a noreply address and uses it for web-based operations automatically)
• A plain sentence on pseudonymity: pick any username, that name is all readers ever see; posting is as anonymous as a pseudonym makes it, no real name or email is displayed
• Keep the existing register and length discipline — this is a fact card, not a tutorial page

Acceptance

• en/ar/fr how-to card covers account creation, private email, pseudonymous posting
• No anonymity overclaim; email-privacy step matches GitHub's actual Settings → Emails flow
• astro check clean, build clean, 390px zero overflow on all three locales