[DOC-9615] Update docs for certificate auth

[mdlinville]

[DOC-9615] Update docs for certificate auth

• Remove duplicate certificate line
• Improve "Supported authentication methods" heading
• HBA configuration syntax section
  • Fix heading level of section
  • Define what HBA stands for
  • Improve the explanations of the example rules to be a bit less informal
• Update the table source
  • Use variables for headings
  • Use reference-style links for in-table links to reduce the column widths
  • Remove redundant "support" from column headings
  • Improve the ordering of the rows

Backported through v23.1

Tests:

• [x] Local build
• [x] Local linkcheck

Previews: src/current/v23.1/security-reference/authentication.md src/current/v23.2/security-reference/authentication.md src/current/v24.1/security-reference/authentication.md src/current/v24.2/security-reference/authentication.md src/current/v24.3/security-reference/authentication.md

[github-actions[bot]]

Files changed:

• src/current/v23.1/security-reference/authentication.md
• src/current/v23.2/security-reference/authentication.md
• src/current/v24.1/security-reference/authentication.md
• src/current/v24.2/security-reference/authentication.md
• src/current/v24.3/security-reference/authentication.md

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[mdlinville]

I'll go ahead and merge this low-risk high-reward change. For v24.3 GA we will need to add LDAP as a supported auth method, but that is tracked via a product change issue.