Improvements to "Compare with existing solutions" section

[olenagerasimova]

I'd suggest to add some improvements to Compare with existing solutions sections

• add some basic info about products we are comparing artipie with: Artifactory and Nexus, at least provide links
• why are we comparing with these solutions and not another ones? May be we need to provide some proof that these are the most commonly used ones
• list of the supported repositories of Nexus seems to be not full: where is Maven? May be we should get list from here

[0crat]

@olenagerasimova/z I'm not managing this repo, remove the webhook or contact me in Slack, as explained in §11; I'm not managing artipie/white-paper GitHub repository, you have to contact me in Slack first, see §11 /cc @yegor256/z