Document environment variables, fixes #772

Some things to discuss:

1. There are some envs/features, which are currently only deployed in the v3 path. Do we need them in v5 (I don't think so - maybe to specify a gateway to debug one...)
2. Can you please check the envs LEAP_PRIVATE_BRIDGE_CERT and LEAP_PRIVATE_BRIDGE. For me they don't make sense. So far I haven't looked into the obfs4 code part

changed milestone to %2024.06 LEAP VPN Release

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(The diff looks weirder that it is..)

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requested review from @jkito

The rendered version looks weird, https://0xacab.org/leap/bitmask-vpn/-/blob/document-envs/docs/debug.rst?ref_type=heads

can you please make sure that the formatting is correct, thanks!

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