Preserve README info of collectors

[Ancairon]

Description

due to the lack of issue template options on netdata/netdata, let's track this here.

as tasks come up they will be added here.

### Tasks
- [ ] xenstat: @thiagoftsm debbugging section missing https://github.com/netdata/netdata/pull/15950
- [x] slabinfo: @thiagoftsm add the note about official native packages, probably in the prerequisites section as an item https://github.com/netdata/netdata/pull/15951
- [x] tor: @MrZammler add the missing requirements to the prerequisites from the README https://github.com/netdata/netdata/pull/15945
- [x] tomcat: @Ancairon fix a typo in the prerequisites https://github.com/netdata/netdata/pull/15982
- [ ] https://github.com/netdata/learn/issues/1791
- [x] samba: @Ancairon should say that the plugin is disabled by default and how to enable it, probably in prerequisites section https://github.com/netdata/netdata/pull/15982
- [x] nsd: @Ancairon add nsd version prerequisite https://github.com/netdata/netdata/pull/15982
- [x] megacli: @tkatsoulas fix commands not in backticks and should say that the plugin is disabled by default and how to enable it, probably in prerequisites section https://github.com/netdata/netdata/pull/16019
- [x] hpssa: @MrZammler should say that the plugin is disabled by default and how to enable it, probably in prerequisites section https://github.com/netdata/netdata/pull/15945
- [x] hddtemp: @Ancairon check details in configuration options https://github.com/netdata/netdata/pull/15982
- [x] go_expvar: @MrZammler should say that the plugin is disabled by default and how to enable it, probably in prerequisites section https://github.com/netdata/netdata/pull/15945
- [ ] https://github.com/netdata/learn/issues/1792
- [ ] https://github.com/netdata/learn/issues/1793
- [x] adaptec_raid: @tkatsoulas make some code text into code blocks, state that the plugin is disabled by default and how to enable it (probably put it in prerequisites) https://github.com/netdata/netdata/pull/16019
- [x] perf: @Ancairon prerequisites section about official rpm packages, fact that it is disabled by default, command keywords, debugging https://github.com/netdata/netdata/pull/15982
- [ ] nfaact: @thiagoftsm add debugging section and prerequisites
- [x] freeipmi: @thiagoftsm prerequisites need to be fixed https://github.com/netdata/netdata/pull/15952 @Ancairon to follow up, as some stuff is missing from the README still https://github.com/netdata/netdata/pull/15955
- [x] @sashwathn we talked about making https://github.com/netdata/netdata/blob/master/exporting/WALKTHROUGH.md a blog https://github.com/netdata/blog/pull/318
- [ ] Asterisk and k6 need metadata.yaml files - @ilyam8
- [x] @sashwathn and if needed @Ancairon excess content from apps.plugin and the likes should go also in either system, application and container metrics pages, which will be present on Learn https://github.com/netdata/blog/pull/318
- [ ] @ilyam8 update apps.plugin and cgroups.plugin metadata yamls
- [x] @thiagoftsm update tc plugin, `Finally, create /etc/netdata/tc-qos-helper.conf with this content: tc_show="class"` bit is missing, and probably should be inside the setup https://github.com/netdata/netdata/pull/16068