docs: Add RPM repository guide for CentOS 7

[imjoey]

Signed-off-by: imjoey majunjie@apache.org

What this PR does / why we need it:

Happy to introduce the Apache APISIX RPM repository, currently only CentOS 7 is supported. This PR is going to add the usage guide for users. Hope this repository will help users to ease the installation.

Currently apisix-build-tools has already used this repository for building the apisix and apisix-base RPMs. Please see the successfully finished CI jobs at https://github.com/api7/apisix-build-tools/pull/111/checks?check_run_id=3902830223 and https://github.com/api7/apisix-build-tools/pull/111/checks?check_run_id=3902830222. Once this PR gets merged, I would be commit other PRs to use this repository in more cases.

Any feedback is much appreciated.

Noted: The repositories for other OS distributions, like Fedora/Ubuntu, will be also supported in the near future.

Pre-submission checklist:

• Did you explain what problem does this PR solve? Or what new features have been added?
• Have you added corresponding test cases?
• Have you modified the corresponding document?
• Is this PR backward compatible? If it is not backward compatible, please discuss on the mailing list first