[BN-1487]: Consolidate Docs into Main Portal

[admiraladmirable]

Purpose

Currently, we have our repo specific documentation living alongside each of the product repos.

https://topl.github.io/Bifrost/
https://topl.github.io/brambl-cli/
https://topl.github.io/BramblSc

The motivation for this is to encourage code submissions to also include documentation updates.

We run into the issue where this docs portal gets out of date quickly, so this PR introduces the remote content plugin to fetch the markdown from each of these repos and include links to the product repos with more information.

This also includes the branding update for Apparatus.

Approach

• Implement https://github.com/rdilweb/docusaurus-plugin-remote-content to download high level guides for each of the getting started tiles.
• Remove outdated documentation.
• Add each downloaded file to .gitignore since they are pulled on each deployment.
• Update branding for Apparatus.

Misc changes

• Update Docusaurus to v3.
• Add direnv files for reproducible Nix environments.
• Delete outdated API docs (these should live in the repo docs if necessary).
• Add PR template and codeowners.
• Add first pass of repository_displatch GitHub action so we can eventually trigger a doc rebuild when docs are updated in the child repos. (not currently implemented, but I can only test this if the repository_dispatch workflow exists in the main branch).

Testing

Built and deployed locally.

Tickets

• BN-1487