[update] add Native Bridge description

[apenzk]

Native Bridge description

Issue

In the mainnet description the Native Bridge is not included

Improvement

The following is proposed:

• add a new Native Bridge section in the Mainnet category of the über-category "Learn"
• add content on "how to bridge" as its focus is user facing
• add a small architecture section (or link to the relevant MIP once the homepage for MIPs is live)

Alternatives:

• the usage of the bridge should be in a different über-category like "API"