Detail the token migration for each forge

Renaming `OTA_ENGINE_GITHUB_TOKEN` would remove the token the engine still
uses to publish datasets to GitHub releases, which contradicted the
following sentence. On GitLab, the engine now publishes datasets with
`OTA_ENGINE_GITLAB_RELEASES_TOKEN` only, so `OTA_ENGINE_GITLAB_TOKEN` can be
renamed, as described in OpenTermsArchive/engine#1287.

Author: clementbiron

content/collections/how-to/report-tracking-failures.md

@@ -76,6 +76,8 @@ Up to engine v16 included, the reporter was part of the engine and configured un
 
 1. Install the module as described above.
 2. Move the `type`, `repositories`, `baseURL` and `apiBaseURL` entries from `@opentermsarchive/engine.reporter` to `@opentermsarchive/issue-reporter`, and add the `collectionApi.url` entry. The legacy `githubIssues` form is not supported.
-3. Rename `OTA_ENGINE_GITHUB_TOKEN` to `OTA_ISSUE_REPORTER_GITHUB_TOKEN`, or `OTA_ENGINE_GITLAB_TOKEN` to `OTA_ISSUE_REPORTER_GITLAB_TOKEN`, in the `.env` file. The engine still needs its own token to publish datasets.
+3. Provide the token of the forge to the module in the `.env` file:
+   - On GitHub, add `OTA_ISSUE_REPORTER_GITHUB_TOKEN`, with the same token as `OTA_ENGINE_GITHUB_TOKEN` or a dedicated one. Keep `OTA_ENGINE_GITHUB_TOKEN` if the engine publishes datasets to GitHub releases, as it still uses it to do so.
+   - On GitLab, rename `OTA_ENGINE_GITLAB_TOKEN` to `OTA_ISSUE_REPORTER_GITLAB_TOKEN`, as the engine no longer uses it. The engine publishes datasets to GitLab releases with `OTA_ENGINE_GITLAB_RELEASES_TOKEN`.
 
 The issues are identified by their title, which did not change, so the module takes over the issues opened by the engine.