Add miner_ endpoints to docs

[fedekunze]

Just noticed that the miner namespace endpoints have not been included on the documentation.

• Add missing endpoints on endpoints.md
• Add link to miner endpoints on namespaces.md

cc: @ramacarlucho @hanchon

[hanchon]

@fedekunze we added a fix when reading the configuration file, because the function that the RPC calls were using was ParseConfig that was returning the node's default configuration instead of the current configuration.

This change makes the 2 miner rpc call work fine, because the setEtherbase function could not read the default min gas because the default MinGasPrice was set to "" (it is using the cosmos-sdk default values). And the SetGasPrice function was saving the new value OK to disk but was overwriting the configuration manually set by the user.