update indexer docs

[iskay]

since namadexer is no longer maintained,

• replaced contents of 'using the indexer' section with heliax namada-indexer
• moved namadexer to a new page marked as deprecated

[iskay]

or should we just remove namadexer section entirely?

[Fraccaman]

we can leave namadexer, but we should also mention about undexer (just a link to their repo is fine)