Merge pull request #1070 from amandafrench/doi-plugin-toc

Adds TOCs to DOI Plugin guide

doi-plugin/en/chapter1.md

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+---
+title: Digital Object Identifiers
+showPageTOC: true
+---
+
 # Digital Object Identifiers
 
 ## What Are DOIs?

doi-plugin/en/doi-plugin.md

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+---
+title: DOI Plugin
+showPageTOC: true
+---
+
 # DOI Plugin
 
 The DOI Plugin assigns DOIs to publications. It can be configured to assign DOIs automatically or manually. The plugin will also generate and display the DOIs on the publication's landing page.  

doi-plugin/en/doi-registration.md

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+---
+title: DOI Registration
+showPageTOC: true
+---
+
 # DOI Registration
 
 Even when a DOI has been assigned to an object in OJS it is not automatically known to the registration agency and the corresponding URL at [https://dx.doi.org/](https://dx.doi.org/) will not yet be resolved. You'll have to register the DOI with one of the DOI registration agencies first. At the time of writing, OJS supports registration with Crossref, mEDRA and DataCite out-of-the-box. You pay a membership fee to register your journal with one of these agencies and  sometimes a small fee per DOI registered.  Some agencies are more suitable for certain types of journals and their prices vary. It is also possible that an organization you already belong to provides membership in one of these agencies.

doi-plugin/en/when-are-dois-assigned.md

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+---
+title: When Are DOIs Assigned?
+showPageTOC: true
+---
+
 # When Are DOIs Assigned?
 
 It is important to understand when DOIs will be assigned because **DOI registration is irreversible**. Once an article or issue is published and your DOI has been registered with an agency, you cannot change it. This is due to the nature of permanent IDs: they are meant to be assigned once and only once and then never change again.