Answering my own question:

The solution that I found was to create a module to deal with "GET" requests of "/".

-module(mod_index).
-export([do/1]).
-include_lib("inets/include/httpd.hrl").

do(ModData) ->
    root_url(ModData#mod.method, ModData#mod.request_uri, ModData#mod.data).

root_url("GET", "/", _) ->
    {proceed, [{response, {200,"Content-Type: text/html\r\n\r\nSomething"}}]};

root_url(_, _, OldData) ->
    {proceed, OldData}.

And adding the module into the configuration file with the default modules:

[{port, 443}, 
 {server_name, "example.com"},
 {server_root, "./root/"},
 {document_root, "./htdocs/"},
 {socket_type, {essl, [{certfile, "/etc/letsencrypt/live/example.com/cert.pem"}, {keyfile, "/etc/letsencrypt/live/example.com/privkey.pem"}]}},
 {directory_index, ["index.html"]},
 {erl_script_alias, {"/erl",[functions]}},
 {erl_script_nocache, true},
 {script_alias, {"/cgi-bin/", "/home/example/site/cgi-bin/"}},
 {script_nocache,true},
 {modules, [mod_index, mod_alias, mod_auth, mod_esi, mod_actions, mod_cgi, mod_dir, mod_get, mod_head, mod_log, mod_disk_log]}
].

As the documentation says, the modules' order matters.