docs: fix link to stable versions

The "scm_raw_web" and "scm_web" macros are used in the documentation to create
links to specific versions of the documents. It's useful to point to the right
version of the examples for instance.

There is a special "stable" version in readthedocs that points to the highest
release, generated links to that version are wrongly pointing to a non existing
"satble" release. This commit fixes that logic by getting the highest release
by using the Github API when building documents for the stable release.

Removed set_preferred_meter_provider_implementation (open-telemetry#497)

Co-authored-by: Mauricio Vásquez <mauricio@kinvolk.io>
Co-authored-by: Chris Kleinknecht <libc@google.com>

Removed set_preferred_meter_provider_implementation (open-telemetry#497)

Co-authored-by: Mauricio Vásquez <mauricio@kinvolk.io>
Co-authored-by: Chris Kleinknecht <libc@google.com>

Changelog and version updates for 0.5b0 release (open-telemetry#498)

Co-authored-by: Chris Kleinknecht <libc@google.com>
Co-authored-by: Leighton Chen <lechen@microsoft.com>

Fix new ext READMEs (open-telemetry#444) (open-telemetry#461)

Version bump for 0.4a0 release (open-telemetry#442)

Fix PR number typo in changelog

Update README for alpha release (open-telemetry#189)