Fix missing doxygen pages

[c1728p9]

Description

Set the doxygen option HIDE_UNDOC_CLASSES to NO so even classes without top level documentation show up. This fixes missing class links in the documentation.

This problem was introduced when the local doxygen settings were synchronized with the website doxygen settings:
784fa91
"Update doxygens to be in sync"

Pull request type

[x] Fix
[ ] Refactor
[ ] Target update
[ ] Functionality change
[ ] Docs update
[ ] Test update
[ ] Breaking change

[c1728p9]

@cmonr this fixes doxygen problems and will need to be backported to 5.10.

[cmonr]

@c1728p9 So a patch release?

[AnotherButler]

Yes, please. Those class references won't appear in the 5.10 Doxygen until this is merged.

[cmonr]

/morph build

[cmonr]

CI started, but will still need a review from someone in Docs to give an "accept" on the PR.

[mbed-ci]

Build : SUCCESS

Build number : 3530
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/8624/

Triggering tests

/morph test
/morph export-build
/morph mbed2-build

[mbed-ci]

Exporter Build : SUCCESS

Build number : 3145
Build artifacts/logs : http://mbed-os.s3-website-eu-west-1.amazonaws.com/?prefix=builds/exporter/8624/

[mbed-ci]

Test : FAILURE

Build number : 3317
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8624/3317

[cmonr]

/morph test

[mbed-ci]

Test : SUCCESS

Build number : 3318
Test logs :http://mbed-os-logs.s3-website-us-west-1.amazonaws.com/?prefix=logs/8624/3318