docs(addition): add mantis tab & docs & fix links

[JafarAz]

Required for merge:

• pr-workflow-check / draft-release-check is ✅ success
• Other rules GitHub shows you, or can be read in configuration

Makes review faster:

• PR title is my best effort to provide summary of changes and has clear text to be part of release notes
• I marked PR by misc label if it should not be in release notes
• Linked Zenhub/Github/Slack/etc reference if one exists
• I was clear on what type of deployment required to release my changes (node, runtime, contract, indexer, on chain operation, frontend, infrastructure) if any in PR title or description
• Added reviewer into Reviewers
• I tagged(@) or used other form of notification of one person who I think can handle best review of this PR
• I have proved that PR has no general regressions of relevant features and processes required to release into production
• Any dependency updates made, was done according guides from relevant dependency
• Clicking all checkboxes
• Adding detailed description of changes when it feels appropriate (for example when PR is big)