add data-structures dir with rlp page

[jmcook1186]

Description

This PR adds a "data-structures" directory to /developers/docs/ and initializes with recursive-length prefix (RLP) material migrated over from eth.wiki. I added a basic landing page at /developers/docs/data-structures/index.md with placeholders for other pages soon to be migrated across. For the RLP page itself (/developers/docs/data-structures/rlp/index.md the information is migrated from eth.wiki almost verbatim except for removing dead links, adding two introductory sentences and bringing the formatting in line with other ethereum.org pages.

The data-structures directory can be a bucket for other related materials to be migrated over from eth.wiki and ethdocs.org that do not yet have a sensible home on ethereum.org.

update: 30th March 22 I have now added pages on SSZ and Patricia Merkle Tries to the data structures directory. The material on SSZ is new, the material on Patricia Merkle tries is migrated almost verbatim from eth.wiki except for very minor addition to make clear these structures are ubiquitous on the execution layer and formatting for ethereum.org. All new material is added to the site menus.

The additions look like this in the dir tree:

developer/docs
           |
           |----data-structures
           |            |--- index.md
          ...           |
                        |--- rlp
                        |     |---index.md
                        |
                        |--- patricia-merkle-trie
                        |     |--- index.md
                        |
                        |--- ssz
                              |---index.md

Related Issue

This resolves suggested PR 10 from #5690 and will enable several other PRs from #5690 and #5731.

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[minimalsm]

@jmcook1186 this is looking great. Mostly nitpicks or site conventions I've highlighted. I've suggested simplifying the language in places or using a more active tone. Feel free to disregard what you don't find appropriate/correct or ask for any clarification :-)

[s1na]

I had a look at the MPT section, but not yet at RLP and SSZ. Had some nits but generally looks good

[wackerow]

@jmcook1186 Sorry for delays reviewing this, but nice work! This looks great overall, and with a couple small adjustments I think we can get this in soon.

In summary:

• I agree we should consider expanding this bucket from "Data structures" to perhaps "Data structures and encoding"
• Just need to clean up the path and links to make sure we use either patricia-merkle-trie or patricia-merkle-tries
• Couple small typos I left fix suggestions for

Otherwise I feel like this is ready to be brought in and any additional fine-tuning can be raised as issues.

[s1na]

I worked through the RLP page and some of the SSZ page. It might be easier to break them each into a PR. MPT and RLP are mostly good to go IMO but SSZ might need some more review.

Also something else is: specially the MPT and RLP pages seem to have as their audience people wanting to implement them. They're detailed explanations of each. I think there's also room for a more high-level explanation for people wanting to use them, maybe with some examples from JS or Python libraries. I don't mean for this PR, but in general.

[minimalsm]

Broken link in sidebar https://ethereumorgwebsitedev01-jmcook1186ethereumorgwebsiterl.gtsb.io/en/developers/docs/data-structures/patricia-merkle-tries/

[minimalsm]

Couple of broken links to fix that I'll come back to later today, but other than that this is good to merge 👍

[minimalsm]

Broken link