Update `$(TOP)/*.md` documents

I updated the top documents to the latest status:

  - HACKING.md:
    - Modify Phabricator to GitLab infomation
    - Remove old Trac information
    - Add link to GitLab activity

  - MAKEHELP.md:
    - Add link to hadrian wiki
    - Fix markdown format

  - INSTALL.md:
    - Modify boot command to remove python3
    - Fix markdown format

  - README.md:
    - Modify tarball file suffix
    - Fix markdown format

I checked the page display on the GitHub and GitLab web.

[skip ci]