chore: Rename master to main #blacklivesmatter (EbookFoundation#6627)

* chore: Rename `master` to `main` #blacklivesmatter

Resolves EbookFoundation#5803

* docs(contributing): fix broken ref link to `programming-playgrounds`

Fix over EbookFoundation#5449 and commit 3c42c36

(cherry picked from commit 59ebfe3)

* fix: Complete EbookFoundation#6614 upstream/main merge

Over commit: EbookFoundation@4d3c9fe

Co-authored-by: ImVector <[email protected]>

Co-authored-by: ImVector <[email protected]>

Author: davorpa

Diff for: .github/PULL_REQUEST_TEMPLATE.md

@@ -11,7 +11,7 @@ Add resource(s) | Remove resource(s) | Add info | Improve repo
 ### For book lists, is it a book? For course lists, is it a course? etc.
 
 ## Checklist:
-- [ ] Read our [contributing guidelines](https://github.com/EbookFoundation/free-programming-books/blob/master/CONTRIBUTING.md)
+- [ ] Read our [contributing guidelines](https://github.com/EbookFoundation/free-programming-books/blob/main/docs/CONTRIBUTING.md)
 - [ ] Search for duplicates.
 - [ ] Include author(s) and platform where appropriate.
 - [ ] Put lists in alphabetical order, correct spacing.

Diff for: docs/CONTRIBUTING-es.md

@@ -257,4 +257,4 @@ Si es posible imprimirlo y conservar su esencia, no es un Tutorial Interactivo.
 [in_process]: #in_process
 [archived]: #archived
 [markdown_guide]: https://guides.github.com/features/mastering-markdown/
-[programming_playgrounds_list]: (https://github.com/EbookFoundation/free-programming-books/blob/master/more/free-programming-playgrounds.md)
+[programming_playgrounds_list]: https://github.com/EbookFoundation/free-programming-books/blob/main/more/free-programming-playgrounds.md

Diff for: docs/CONTRIBUTING.md

@@ -2,7 +2,7 @@
 
 
 ## Contributor License Agreement
-By contributing you agree to the [LICENSE](https://github.com/EbookFoundation/free-programming-books/blob/master/LICENSE) of this repository.
+By contributing you agree to the [LICENSE](https://github.com/EbookFoundation/free-programming-books/blob/main/LICENSE) of this repository.
 
 ## Contributor Code of Conduct
 By contributing you agree to respect the [Code of Conduct](CODE_OF_CONDUCT.md) of this repository. ([translations](../README.md#translations))