docs: restructure help pages into bundles and add new reference pages

Convert several pages to leaf bundles with WebP screenshots, add
api-keys, domain-availability, notifications, provider-features,
quotas, services, settings, signup and subdomains pages, and refresh
existing content.

content/pages/signup.en.md

@@ -0,0 +1,44 @@
+---
+date: 2026-06-10T12:00:00+02:00
+title: Create an account
+author: nemunaire
+weight: 100
+description: "Sign up for a happyDomain account and validate your email address"
+---
+
+Before you can manage your domains, you need a happyDomain account. Creating one takes less than two minutes and only requires an email address and a password.
+
+## Filling in the sign-up form
+
+From the home page, follow the link to register, then fill in the form:
+
+<!-- TODO: screenshot of the sign-up form -->
+
+- **Email address** -- this address identifies you on the platform and is used to contact you for security-related operations (password recovery, important notices). It must contain a valid `@`.
+- **Password** -- choose a strong password (see below).
+- **Password confirmation** -- retype the exact same password to avoid typos.
+- **Keep me informed of future big improvements** -- an optional checkbox to subscribe to the project newsletter. Leave it unchecked if you do not wish to receive these updates.
+
+The interface language you are currently using is automatically recorded with your account, so the emails you receive will be in that language.
+
+{{% notice style="tip" title="Password requirements" icon="key" %}}
+A password is accepted only if it contains at least 8 characters, including at least one uppercase letter, one lowercase letter and one digit. It must also contain a special character, or be at least 11 characters long.
+{{% /notice %}}
+
+Some servers display an anti-bot challenge (captcha) below the form. If so, complete it before submitting.
+
+When everything is filled in correctly, click **Sign up!**.
+
+## Validating your email address
+
+On most servers, an email is sent to the address you provided right after registration. Open your mailbox and click the validation link it contains to activate your account.
+
+<!-- TODO: screenshot of the email validation page -->
+
+If you did not receive the message (check your spam folder first), you can request a new one from the email validation page by entering your address again and clicking **Send again**.
+
+{{% notice style="info" title="Servers without email" icon="circle-info" %}}
+Some happyDomain instances run without an email service. On these servers, no validation message is sent: your account is usable immediately and you can [log in]({{% relref "login" %}}) right away.
+{{% /notice %}}
+
+Once your address is validated, you are redirected to the [login page]({{% relref "login" %}}) where you can sign in with your new credentials.