[DOCS] massive documentation update (#20229)

This PR:

- reorganizes all documentation pages so they live in the right category
- removes lots of legacy docs
- contains many improvements to active documentation pages

Geth user documentation is now spread across five major categories:

- Install and Build: installation and compile instructions
- Using Geth: this is for pages about general geth usage.
- For dApp Developers: this is for programming guides and functionality specific
   to dapp development. All the dev guides for mobile framework and Go APIs live here.
- JSON-RPC APIs: this has its own section because there is now a sub-page for
   every name space. I have also added an overview text that explains how to set
   up the API servers.
- For Geth Developers: this is for geth contributors

docs/_legacy/Gas-Price-Oracle.md

@@ -0,0 +1,44 @@
+---
+title: Gas price oracle
+---
+
+The gas price oracle is a helper function of the Geth client that tries to find an
+appropriate default gas price when sending transactions. It can be parametrized with the
+following command line options:
+
+- `gpomin`: lower limit of suggested gas price. This should be set at least as high as the
+  `gasprice` setting usually used by miners so that your transactions will not be rejected
+  automatically because of a too low price.
+
+- `gpomax`: higher limit of suggested gas price. During load peaks when there is a
+  competition between transactions to get into the blocks, the price needs to be limited,
+  otherwise the oracle would eventually try to overbid everyone else at any price.
+
+- `gpofull`: a block is considered "full" when a certain percentage of the block gas limit
+  (specified in percents) is used up by transactions. If a block is not "full", that means
+  that a transaction could have been accepted even with a minimal price offered.
+
+- `gpobasedown`: an exponential ratio (specified in `1/1000ths`) by which the base price
+  decreases when the lowest acceptable price of the last block is below the last base
+  price.
+
+- `gpobaseup`: an exponential ratio (specified in `1/1000ths`) by which the base price
+  increases when the lowest acceptable price of the last block is over the last base
+  price.
+
+- `gpobasecf`: a correction factor (specified in percents) of the base price. The
+  suggested price is the corrected base price, limited by `gpomin` and `gpomax`.
+
+The lowest acceptable price is defined as a price that could have been enough to insert a
+transaction into a certain block. Although this value varies slightly with the gas used by
+the particular transaction, it is aproximated as follows: if the block is full, it is the
+lowest transaction gas price found in that block. If the block is not full, it equals to
+gpomin.
+
+The base price is a moving value that is adjusted from block to block, up if it was lower
+than the lowest acceptable price, down otherwise. Note that there is a slight amount of
+randomness added to the correction factors so that your client will not behave absolutely
+predictable on the market.
+
+If you want to specify a constant for the default gas price and not use the oracle, set
+both `gpomin` and `gpomax` to the same value.