[STAR-2903] Update information about sustainability dashboard

Follows up on BB-6571 / !44 (merged) and updates information about sustainability dashboard in README to fully match current processes.

It also fixes a (pretty crucial 😅) typo about what the sustainability dashboard displays.

Tickets

• STAR-2903

Related PRs

• opencraft/documentation/public!483 (merged)

Test instructions

• Read over diff and check out rendered changes here.

Author notes

• Strictly speaking, frontend code would need to be extended so that headers of sustainability table displayed on user page link to new sections of README describing what stats mean at user level. This is out of scope for the ticket that prompted this PR, though, and should be done via a follow-up task.

Reviewers

• @Agrendalath