Issue: #345

This PR primarily adds new documentation pages covering generic integration with DejaCode using the REST API and Webhooks.
It also includes a few code changes to extend these capabilities, ensuring they offer the same feature support as the existing integration modules (Jira, GitHub, etc.).

Documentation addition:

• New page: Integrations overview
• New page: REST API Integration
• New page: Webhook for integrations
• New entry in the FAQ: How can I integrate DejaCode with my tools or applications?

Changes:

• Add the sync method on BaseIntegration
• Add the ability to add Request comment in REST API
• Remove the obligation to provide an assignee in REST API context

Framework:

The workflow.integrations.base.BaseIntegration class serves as the core framework
for the five fully supported platforms: Jira, GitHub, GitLab, SourceHut,
and Forgejo.

It centralizes the code and logic shared across these integrations to maximize
reusability and consistency. This includes handling HTTP requests, maintaining a
session, managing logging, and generating the request title and
content.

To add support for a new platform, create a subclass of BaseIntegration and
implement the following methods:

• get_headers – Returns the authentication headers required by the target platform.
• sync – Creates or updates an external issue to reflect the given request.
• post_common – Posts a comment to an external issue.