[doc] Add notes on how to activate bash completion for locally built binaries

Summary:
This does not appear to be officially supported (see https://www.gnu.org/software/bash/manual/html_node/Programmable-Completion.html) but works well enough on Debian that its worth sharing.

I purposefully did not amend the ./contrib/*.bash-completion files to include
relative paths to binaries since this may have negative impacts for some users.
For example, ./bitcoin-cli would match for any bitcoin software fork that has
not renamed the bitcoin-cli binary. This could result in completions applied
to the wrong binaries. It is best to leave this decision to the user since they
can source different .bash-completion files only in the appropriate session.

Test Plan:

source ./contrib/bitcoind.bash-completion
complete -F _bitcoind ./bitcoind
mkdir build && cd build && cmake -GNinja .. && ninja
./src/bitcoind -testnet -daemon   # type this out using tab-completion
cd src
./bitcoin-cli -testnet   # type this out using tab-completion, hit space then
                         # tab again to see RPC commands

Note: bitcoind must be running in order for RPC command tab completion to work.
This is expected.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D12965