Fix rendering of domain management docs, note table of contents

[NelsonVides]

mkdocs-material loses the table of contents if we start from the top index (single #).

[codecov]

Codecov Report

Merging #3162 (114a536) into master (f8218b2) will increase coverage by 2.80%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #3162      +/-   ##
==========================================
+ Coverage   77.41%   80.22%   +2.80%     
==========================================
  Files         396      396              
  Lines       32306    32306              
==========================================
+ Hits        25010    25916     +906     
+ Misses       7296     6390     -906     

Impacted Files	Coverage Δ
src/domain/mongoose_domain_core.erl	88.09% <0.00%> (-1.20%)	⬇️
src/ejabberd_sm.erl	84.26% <0.00%> (-0.33%)	⬇️
src/mod_roster.erl	79.41% <0.00%> (-0.25%)	⬇️
src/mod_muc_room.erl	76.92% <0.00%> (-0.18%)	⬇️
src/mod_muc_log.erl	77.77% <0.00%> (ø)
src/ejabberd_c2s.erl	89.27% <0.00%> (+0.22%)	⬆️
src/mod_muc.erl	75.00% <0.00%> (+0.22%)	⬆️
src/ejabberd_s2s_out.erl	59.45% <0.00%> (+0.45%)	⬆️
src/mam/mod_mam_utils.erl	89.96% <0.00%> (+1.00%)	⬆️
src/domain/service_domain_db.erl	79.68% <0.00%> (+1.56%)	⬆️
... and 13 more

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[arcusfelis]

a