Add ToC to larger docs, like site/fluxctl

[dholbach]

In #1178 (comment) @hiddeco suggested to add a ToC to some of the larger docs, like site/fluxctl.md.

Looking at our docs, I see fluxctl.md and faq.md as obvious first targets:

[daniel@reef flux ]$ find . -iname '*.md' | egrep -v "^.\/vendor\/" | xargs wc -l | sort -n -k1 -r
  4344 insgesamt
   606 ./site/fluxctl.md                              <-----------
   571 ./CHANGELOG.md
   429 ./site/faq.md                                  <-----------
   306 ./site/helm-integration.md
   260 ./site/upgrading-to-1.0.md
   255 ./chart/flux/README.md
   224 ./site/helm-operator.md
   194 ./CHANGELOG-helmop.md
   189 ./site/helm-get-started.md
   180 ./site/standalone-setup.md
   153 ./site/helm-upgrading-to-beta.md
   140 ./internal_docs/releasing.md
   138 ./site/get-started.md
   121 ./README.md
    97 ./site/how-it-works.md
    93 ./CONTRIBUTING.md
    84 ./site/daemon.md
    74 ./site/troubleshooting.md
    68 ./site/introduction.md
    43 ./site/building.md
    35 ./site/requirements.md
    26 ./site/installing.md
    23 ./site/monitoring.md
    23 ./.github/PULL_REQUEST_TEMPLATE.md
     9 ./chart/README.md
     3 ./CODE_OF_CONDUCT.md
[daniel@reef flux ]$