Merge bitcoin/bitcoin#22235: script: add script to generate example bitcoin.conf

b42643c253 doc: update init.cpp -conf help text (josibake)
970b9987ad doc: update devtools, release-process readmes (josibake)
50635d27b4 build: include bitcoin.conf in build outputs (josibake)
6aac946f49 doc: update bitcoin-conf.md (Josiah Baker)
1c7e820ded script: add script to generate example bitcoin.conf (josibake)
b483084d86 doc: replace bitcoin.conf with placeholder file (josibake)

Pull request description:

  create a script for parsing the output from `bitcoind --help` to create an example conf file for new users

  ## problem

  per #10746 , `bitcoin.conf` not being put into the data directory during installation causes some confusion for users when running bitcoin. in the discussion on the issue, one proposed solution was to have an example config file and instruct users to `cp` it into their data directory after startup. in addition to #10746 , there have been other requests for a "skeleton config file" (https://github.com/bitcoin/bitcoin/issues/19641) to help users get started with configuring bitcoind.

  the main issue with an example config file is that it creates a second source of truth regarding what options are available for configuring bitcoind. this means any changes to the options (including the addition or removal of options) would have to be updated for the command line and also updated in the example file.

  this PR addresses this issue by providing a script to generate an example file directly from the `bitcoind --help` on-demand by running `contrib/devtools/gen-bitcoin-conf.sh`. this solution was originally proposed on #10746 and would also solve #19641 . this guarantees any changes made to the command-line options or the command-line options help would also be reflected in the example file after compiling and running the script.

  the main purpose of this script is to generate a config file to be included with releases, same as `gen-manpages.sh`. this ensures every release also includes an up-to-date, full example config file for users to edit. the script is also available for users who compile from source for generating an example config for their compiled binary.

  ## special considerations

  this removes the `bitcoin.conf` example file from the repo as it is now generated by this script. the original example file did contain extra text related to how to use certain options but going forward all option help docs should be moved into `init.cpp`

  this also edits `init.cpp` to have the option help indicate that `-conf` is not usable from the config file. this is similar to how `-includeconf` 's help indicates it cannot be used from the command line

ACKs for top commit:
  laanwj:
    Tested and code review ACK b42643c253

Tree-SHA512: 4546e0cef92aa1398da553294ce4712d02e616dd72dcbe0b921af474e54f24750464ec813661f1283802472d1e8774e634dd1cc26fbf1f13286d3e0406c02c09

src/init.cpp

@@ -409,7 +409,7 @@ void SetupServerArgs(ArgsManager& argsman)
     argsman.AddArg("-blockreconstructionextratxn=<n>", strprintf("Extra transactions to keep in memory for compact block reconstructions (default: %u)", DEFAULT_BLOCK_RECONSTRUCTION_EXTRA_TXN), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-blocksonly", strprintf("Whether to reject transactions from network peers. Automatic broadcast and rebroadcast of any transactions from inbound peers is disabled, unless the peer has the 'forcerelay' permission. RPC transactions are not affected. (default: %u)", DEFAULT_BLOCKSONLY), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-coinstatsindex", strprintf("Maintain coinstats index used by the gettxoutsetinfo RPC (default: %u)", DEFAULT_COINSTATSINDEX), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
-    argsman.AddArg("-conf=<file>", strprintf("Specify path to read-only configuration file. Relative paths will be prefixed by datadir location. (default: %s)", BITCOIN_CONF_FILENAME), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
+    argsman.AddArg("-conf=<file>", strprintf("Specify path to read-only configuration file. Relative paths will be prefixed by datadir location (only useable from command line, not configuration file) (default: %s)", BITCOIN_CONF_FILENAME), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-datadir=<dir>", "Specify data directory", ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);
     argsman.AddArg("-dbbatchsize", strprintf("Maximum database write batch size in bytes (default: %u)", nDefaultDbBatchSize), ArgsManager::ALLOW_ANY | ArgsManager::DEBUG_ONLY, OptionsCategory::OPTIONS);
     argsman.AddArg("-dbcache=<n>", strprintf("Maximum database cache size <n> MiB (%d to %d, default: %d). In addition, unused mempool memory is shared for this cache (see -maxmempool).", nMinDbCache, nMaxDbCache, nDefaultDbCache), ArgsManager::ALLOW_ANY, OptionsCategory::OPTIONS);