Clean up GitHub issues

[dwelsch-esi]

Overview

There are dozens of issues open against the Vitess documentation, going back to 2018.
Improve documentation issue handling by cleaning up old issues and implementing procedures.

Audience: Documentation contributors

Type: All

Context

This issue tracks recommended changes resulting from an analysis of the Vitess
documentation commissioned by CNCF. The analysis and supporting documents are
here: https://github.com/cncf/techdocs/tree/main/analyses/0014-vitess.

See the #1955 listing all issues identified in the analysis.

Possible Implementation

Related material in the current doc:

• https://github.com/vitessio/website/issues/

Suggested changes:

• Go through the issues backlog and close or update old issues.
• Create an issue template to ensure that issue descriptions are consistent and
  complete.
• Write a "new contributors" document. Post prominently.