Feat: Add custom styling for blockquote, add Telegram TextDirective

[emiliarepo]

Description

Adds custom styling for blockquotes and a shorthand text directive for Telegram links.

The Telegram shorthand makes adding Telegram links easier (you only have to change the name once), but it also makes the links incompatible with the original ilmomasiina front. I believe we didn't want to add guild-specific features to the original front (and we should in general discourage people from using the original frontend since it doesn't support languages in the description). I think it would be a nice QOL change for event organizers (and I feel like adding small Telegram icons next to the @s would be nice, can add in this PR if we decide to stick with this change), but I also don't have a major issue with dropping this if ilmomasiina compatibility is preferred.

The pull request also updates guild-branded styling for blockquotes to be used for HCP information, signup binding info etc. Opted not to add support for Github Alerts style markdown yet as I don't see too much use for them in a guild context.

Before submitting the PR, please make sure you do the following

• [⚔️] If your PR is related to a previously discussed issue, please link to it here.
• [✅] Prefix your PR title with feat:, fix:, chore:, or docs:.
• [✅] This message body should clearly illustrate what problems it solves.
• [✅] Make sure the commit history is linear, up-to-date with main branch and does not contain any erroneous changes

Formatting and linting

• [✅] Format code with pnpm format and lint the project with pnpm lint

[kahlstrm]

Looks good, but let's have the @types/unist in devDependencies.

[kahlstrm]

This probably should be in devDependencies