feat: support adding custom links on the nav header

[fgalind1]

This PR allows to configure custom links to be added in the menu/navigation header by adding them in the config.toml file

For example:

[[UI.header_links]]
    name = "Confluence"
    icon_url = "https://www.atlassian.com/favicon.ico"
    url = "https://www.atlassian.com"

[[UI.header_links]]
    name = "Issues"
    icon_url = "https://mintlify.s3-us-west-1.amazonaws.com/chainlit-43/_generated/favicon/favicon.ico?v=3"
    url = "https://github.com/Chainlit/chainlit/issues"

That will result in the following:

[willydouhard]

That's great! The size of the button is too big i think. Also we should remove the current github config which does exactly that buy specifically for github, this can be now generalised with this PR

[fgalind1]

That's great! The size of the button is too big i think. Also we should remove the current github config which does exactly that buy specifically for github, this can be now generalised with this PR

indeed, I've adjusted the size, now they show more uniform with the theme/user icons

prev size:

@willydouhard for the github config I only found a reference of being used here https://github.com/Chainlit/chainlit/blob/main/backend/chainlit/server.py#L347 but not to used in the frontend or render an icon/link anywhere?

[fgalind1]

I believe it is indeed not used, so I've created a PR #1865 that removes this in favor of this generalized approach