docs(user-guide): add xwiki data source documentation

[AlexMell]

Summary

Adds end-user documentation for the xWiki data source type (EPMCDME-13142), which lets assistants search and retrieve content from a self-hosted xWiki instance. Matches the depth and structure of the existing Azure DevOps Wiki data source page.

Changes

• New page docs/user-guide/data-source/datasources-types/add-xwiki-data-source.md — step-by-step configuration guide covering prerequisites, all four xWiki-specific form fields (Space, Wiki, Integration for xWiki, Embeddings model), URL format and auth admonitions, What Gets Indexed (page content, 5 000-page limit, failure vs. skipped page distinction), and Using in Assistants
• New FAQs faq/how-do-i-add-an-xwiki-data-source-in-codemie.md and faq/why-does-my-xwiki-integration-return-a-404-error.md
• sidebars.ts — inserted add-xwiki-data-source between add-azure-devops-work-items-data-source and add-provider-datasource
• Pagination chain updated — add-azure-devops-work-items-data-source.md pagination_next now points to the new xWiki page; add-provider-datasource.md pagination_prev updated accordingly
• add-provider-datasource.md pagination fix (intentional) — this file's pagination_prev was stale, pointing to add-azure-devops-wiki-data-source instead of add-azure-devops-work-items-data-source. Inserting the xWiki page into the chain corrects it as a side effect. The change is deliberate and restores the correct linear navigation order.
• docs/user-guide/data-source/index.md — added xWiki entry to the Data Source Types list
• cspell.config.yaml — added xwiki, xWiki, and unindexed to the project dictionary

Testing

• Tested locally with npm start
• All pages render correctly
• Images display properly
• Internal links work
• Sidebar navigation works

Quality Checks

• npm run check passes (typecheck + lint + commitlint)
• No MDX compilation errors
• No raw angle brackets (<text> must be `<text>`)
• Sidebar references document IDs (not filenames)
• Images stored locally next to content (not in static/img/)
• Commit messages follow Conventional Commits
• No secrets or credentials in documentation

Additional Notes

No screenshots available for this feature — page is text-only.