Allow marking translations reviewed

[fflorent]

When machine translations have been modified and reviewed, there is no need anymore to show this warning:

https://github.com/gristlabs/grist-help/blob/master/help/fr/MACHINE-TRANSLATION.md

I propose to add to the mkdocs.yml an extra config to mark translations as being reviewed.

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[dsagal]

Might it be better to put such a marker into each page's meta data (https://www.mkdocs.org/user-guide/writing-your-docs/#yaml-style-meta-data)?

[fflorent]

@dsagal I originally abandoned this idea: Git localize detects these metadata, so it would be expected that this property appears in both English and French (or any other language). I thought it would not be convenient, as the original writers (of the English version) have to keep in mind adding this property in the metadata.

But thinking again, what do you think of introducing a new command that does the following?

• machine-translate any missing page for any language;
• also machine-translate any page that have not been reviewed for the target languages when changes have been made in the English version;
• also add the marker translation_reviewed: true in the English version of the page metadata and translation_reviewed: false in the others (so the admonition would appear);

We could make this run in Github actions every 24h, and/or on push on master, and/or when triggered manually (with a PR if some files have been updated).

The advantages are that:

• the machine translations are nearly immediately available, with no action required;
• the translators would have only the markdown file to update: they review and update the translation and at the end change the value of the marker translation_reviewed, and everything could be done through Git Localize;

The tricky part is that translators should keep track of the changes made in the English documentation (unless we find a way to tell GPT to only detect and translate parts of the documents that have been changed without overriding the existing reviewed translations). Something I still need to think of.

For the scope of this PR, I suggest that I put aside the machine translation and only make this command add the translation_reviewed marker, and use it in some Github action.

[fflorent]

Working on an idea, for now changing it to draft.

[fflorent]

Waiting for #396 to be merged before reopening and working again on this PR