docs(filter): add filter query explanation

docs/content/doc/usage/filters.md

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+---
+title: "Filters"
+date: 2024-03-09T19:51:32+02:00
+draft: false
+type: doc
+menu:
+  sidebar:
+    parent: "usage"
+---
+
+# Filter Syntax
+
+To filter tasks via the api, you can use a query syntax similar to SQL. 
+
+This document is about filtering via the api. To filter in Vikunja's web ui, check out the help text below the filter query input.
+
+{{< table_of_contents >}}
+
+## Available fields
+
+The available fields for filtering include:
+
+*   `done`: Whether the task is completed or not
+*   `priority`: The priority level of the task (1-5)
+*   `percentDone`: The percentage of completion for the task (0-100)
+*   `dueDate`: The due date of the task
+*   `startDate`: The start date of the task
+*   `endDate`: The end date of the task
+*   `doneAt`: The date and time when the task was completed
+*   `assignees`: The assignees of the task
+*   `labels`: The labels associated with the task
+*   `project`: The project the task belongs to (only available for saved filters, not on a project level)
+
+You can date math to set relative dates. Click on the date value in a query to find out more.
+
+## Operators
+
+The available operators for filtering include:
+
+*   `!=`: Not equal to
+*   `=`: Equal to
+*   `>`: Greater than
+*   `>=`: Greater than or equal to
+*   `<`: Less than
+*   `<=`: Less than or equal to
+*   `like`: Matches a pattern (using wildcard `%`)
+*   `in`: Matches any value in a list
+
+To combine multiple conditions, you can use the following logical operators:
+
+*   `&&`: AND operator, matches if all conditions are true
+*   `||`: OR operator, matches if any of the conditions are true
+*   `(` and `)`: Parentheses for grouping conditions
+
+## Examples
+
+Here are some examples of filter queries:
+
+*   `priority = 4`: Matches tasks with priority level 4
+*   `dueDate < now`: Matches tasks with a due date in the past
+*   `done = false && priority >= 3`: Matches undone tasks with priority level 3 or higher
+*   `assignees in [user1, user2]`: Matches tasks assigned to either "user1" or "user2
+*   `(priority = 1 || priority = 2) && dueDate <= now`: Matches tasks with priority level 1 or 2 and a due date in the past
+
+