docs: add file permissions and ownership documentation for claim.conf (#22017)
* docs: add file permissions and ownership documentation for claim.conf This commit adds documentation about required file permissions and ownership for claim.conf, which contains sensitive claiming tokens. Changes: - Add 'File Permissions and Ownership' subsection after 'Applying the Configuration' - Document required permissions: 0640 (owner read/write, group read, no world access) - Document required ownership: root:netdata - Include security rationale for why claim.conf should not be world-readable - Note that the claiming script automatically sets these permissions - Use Docusaurus admonition (:::important) for security note * Apply suggestion from @Ancairon * Update README.md --------- Co-authored-by: nedi-audit <nedi-audit@users.noreply.github.com> Co-authored-by: Fotis Voutsas <fotis@netdata.cloud>