Add a placeholder 0.15 upgrade notification; add reminders about the upgrade notification in release process docs

[jonboiser]

Summary

1. Some edits to release_process.rst, including adding cross links to i18n doc, changing "CrowdIn" to "Crowdin" (to match their own branding)
2. Adds a reminder about the upgrade notification (I am working on more internal docs in Notion and on the kolibri-release-process tool)
3. Adds a placeholder upgrade notification for 0.15.0 with a TODO(i18n) comment to remind us to finalize this once we know the full feature set.

References

Fixes #7452 as far as the Kolibri codebase is concerned. Other documentation work is still being worked on and will be until 0.15 is released.

I would recommend writing a new issue to remind us to come up with a final version of the upgrade message before the string freeze.

Reviewer guidance

1. Are the edits to the docs good? Feel free to fork, edit, and push to this branch!

[radinamatic]

All looks good to me 💯 , I only suggested some styling changes. 🙂

[radinamatic]

Perfect, let's merge this! 👍🏽

[indirectlylit]

thanks for these updates