feat(docs): Update Catalyst Signed Document specification.

[Mr-Leshiy]

Description

• Fixed the part of specifying template fields for proposal, comment and review documents.
• Improved description of the comment's ref, reply fields.
• Added a new category_id field for the proposal document.

[github-actions]

✅ Test Report | ${\color{lightgreen}Pass: 276/276}$ | ${\color{red}Fail: 0/276}$ |

[github-actions]

✅ Test Report | ${\color{lightgreen}Pass: 276/276}$ | ${\color{red}Fail: 0/276}$ |

[saibatizoku]

LGTM

[github-actions]

✅ Test Report | ${\color{lightgreen}Pass: 276/276}$ | ${\color{red}Fail: 0/276}$ |

[bkioshn]

This changes look good to me, but in metadata fields for comment, proposal, review
should this be content-encoding?

"content-type" => "br"

[github-actions]

✅ Test Report | ${\color{lightgreen}Pass: 276/276}$ | ${\color{red}Fail: 0/276}$ |

[github-actions]

✅ Test Report | ${\color{lightgreen}Pass: 276/276}$ | ${\color{red}Fail: 0/276}$ |