API spec HTML: Use Github Pages rather than htmlpreview

htmlpreview.github.io breaks some links: in index.html viewed through
this service, the links that should go to from_doxygen.html are shown
as internal links, so the meat of the document is invisible.

Link to Github pages instead. This relies on the documentation being
available on the `master` branch of the mbed-crypto repository.

This has the added benefit of not depending on a third-party service.

README.md

diff --git a/README.md b/README.md
index 8d9f2de..9d34c55 100644
--- a/README.md
+++ b/README.md
@@ -30,7 +30,7 @@
 
 ### PSA Cryptography API
 
-You can read the [complete PSA cryptography API specification as a PDF document](https://github.com/ARMmbed/mbed-crypto/raw/psa-crypto-api/docs/PSA_Cryptography_API_Specification.pdf). The API reference is also available in [HTML format](https://htmlpreview.github.io/?https://github.com/ARMmbed/mbed-crypto/blob/psa-crypto-api/docs/html/index.html).
+You can read the [complete PSA cryptography API specification as a PDF document](https://github.com/ARMmbed/mbed-crypto/raw/psa-crypto-api/docs/PSA_Cryptography_API_Specification.pdf). The API reference is also available in [HTML format](https://armmbed.github.io/mbed-crypto/html/index.html).
 
 ### Browsable library documentation