Replace Orion with Tessera (#118)

[rolandtyler]

Pull request checklist

Use the following list to make sure your PR fits the EthSigner doc quality standard.

Before creating the pull request

Make sure that:

• you have read the contribution guidelines.
• you have tested your changes locally before submitting them to the community for review.

After creating your pull request and tests finished

Make sure that:

• you have fixed all the issues raised by the tests, if any.
• you have verified the rendering of your changes on
  ReadTheDocs.org PR preview
  and updated the testing link (see Testing).

Describe the change

Replace Orion references and links with Tessera

Issue fixed

Fixes #118

Impacted parts

For content changes

• Doc content
• Doc pages organisation

For tools changes

• CircleCI workflow
• Build and QA tools (lint, vale,…)
• MkDocs templates
• MkDocs configuration
• Python dependencies
• Node dependencies and JavaScript
• ReadTheDocs configuration
• GitHub integration

Testing

Screenshots / recording

[alexandratran]

LGTM.