doc: add public explaination on sharing

docs/guide/sharing.md

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+# Sharing
+
+OxiCloud lets you share any file or folder with other people. Open the
+item, click **Share**, and pick who you'd like to share it with.
+
+## Who you can share with
+
+| Share with | When to use it |
+|---|---|
+| **Another OxiCloud user** | The person already has an account here. |
+| **A group** | A team or department — every member of the group gets access. |
+| **An email address** | The person doesn't have an account yet. We'll send them an invitation by email. |
+| **A public link with password** | Anyone with the link *and* the password can open the file. |
+| **A public link** | Anyone with the link can open the file — no sign-in needed. |
+
+## What they can do
+
+Pick a level when you share. Each level includes everything the level
+above it allows.
+
+| Level | What it allows |
+|---|---|
+| **Can view** | Open and download. |
+| **Can edit** | Plus create, rename, and modify files. |
+| **Can manage** | Plus delete and reshare. |
+
+## Public links are view-only
+
+For safety, public links can only be **view-only**. There's no way to
+know *who* opened a public link, so allowing edits would make it
+impossible to tell who changed what.
+
+If you need to let someone make changes, share with their **email**
+instead. They'll receive an invitation, and from then on every change
+they make is recorded under their name.
+
+## Expiration
+
+When you share, you can set an **expiration date**. After that date,
+the share simply stops working — the recipient sees a "not found" page.
+
+This is especially handy for public links and one-off collaborations.
+
+## Sharing by email
+
+When you enter someone's email address, OxiCloud sends them a message
+with a sign-in link. Clicking that link signs them in and opens the
+file or folder you shared — no password to create, no form to fill in.
+
+The link expires after a day. If they miss it, you can resend it from
+the **My shares** section.
+
+## Keeping track — *My shares*
+
+The **My shares** section in the sidebar lists everything you've
+shared. For each item, you can see:
+
+- Who you shared it with (a person, a group, an email, or a public
+  link)
+- The access level you gave them
+- When the share expires, if ever
+- When it was created
+
+From there you can change the level, change the expiration, or revoke
+the share entirely.
+
+If someone has shared something **with you**, look in **Shared with
+me** instead.
+
+## Quick recipes
+
+**Let a colleague edit a folder, with a record of their changes.**
+Open the folder → *Share* → enter their email → *Can edit* → *Send*.
+
+**Send a one-off read-only link to an external partner.**
+Open the file → *Share* → *Public link* → set a password → set an
+expiration → copy the link.
+
+**Give a whole team access.**
+Open the folder → *Share* → pick the group → *Can view* (or *Can
+edit*).