Add Confluence REST API coverage analysis documentation

[Copilot]

Documents current library implementation against Atlassian's Confluence REST API to identify gaps for future development.

Changes

Created API_COVERAGE.md - Comprehensive analysis covering:

• Current Implementation: Catalogued all 6 domains (Content, Space, User, Group, Attachment, Misc) with method inventories
• Missing Functionality: 69 endpoints across 10 categories including:
  • Content restrictions/permissions, properties, versions, descendants, comments
  • Space permissions, properties, settings, themes
  • User search, properties, bulk operations
  • Group management enhancements
  • New domains: Audit, Longtask, Analytics, Blueprint, Settings
• Deprecations: 6 areas requiring attention (user keys, personal spaces, API v2 migration, FlyingPDF, attachment deletion workarounds)
• Feature Requests: 15 prioritized, actionable requests with API endpoints, implementation guidance, and file references

Each missing feature includes:

• Atlassian API documentation links
• Use cases
• Target files for implementation
• Cloud vs Server considerations

Updated README.md - Added reference to API coverage documentation

Structure

Feature requests formatted for automated implementation:

#### FR-001: Content Restrictions Management
**API Endpoints**:
- `GET /rest/api/content/{id}/restriction`
- `POST /rest/api/content/{id}/restriction`

**Implementation**:
- Create `IContentRestrictionDomain` interface
- Add extension methods in `ContentRestrictionExtensions.cs`
- Create entity classes: `ContentRestriction`, `RestrictionUser`

**Files to Create/Modify**:
- Create: `src/Dapplo.Confluence/ContentRestrictionExtensions.cs`
- Modify: `src/Dapplo.Confluence/IConfluenceClient.cs`

**Reference**: https://developer.atlassian.com/cloud/confluence/rest/v1/...

Enables informed decisions on library expansion and prioritization of 100+ missing API features.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Create a list of functionality which Dapplo.Confluence doesn't support or is obsoleted / removed in the Confluence Endpoints.</issue_title>
<issue_description>As a developer I would like to know what functionality still needs to be added to this library to have a good coverage of that which Atlassian makes possible via their Confluence API.

Additionally, for the current available functionality in this library, I need to know what is no longer supported or still missing fields and information.

Write this down in a markdown file, with links to the documentation or Atlassian and corresponding files in this project.
A bonus would be to have the description in such a way that I can use it to request this as a feature request where Copilot can work on.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Create a list of functionality which Dapplo.Confluence doesn't support or is obsoleted / removed in the Confluence Endpoints. #76

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