Introduce API_PATH env var

[vbaranov]

Motivation

If Blockscot instance is proxied to blockscout.com there are 2 problems exist:

• URL on API docs page doesn't include path on blockscout.com for the instance that confusing developers
• Try it out section doesn't work for ETH RPC API endpoints

Changelog

API_PATH ENV var introduced which indicates the path for API URL. If it is not set, it is /.

Checklist for your Pull Request (PR)

• I added an entry to CHANGELOG.md with this PR
• If I added new functionality, I added tests covering it.
• If I fixed a bug, I added a regression test to prevent the bug from silently reappearing again.
• I checked whether I should update the docs and did so by submitting a PR to https://github.com/blockscout/docs
• If I added/changed/removed ENV var, I submitted a PR to https://github.com/blockscout/docs to update the list of env vars at https://github.com/blockscout/docs/blob/master/for-developers/information-and-settings/env-variables.md and I updated the version to master in the Version column. Changes will be reflected in this table: https://docs.blockscout.com/for-developers/information-and-settings/env-variables.
• If I add new indices into DB, I checked, that they are not redundant with PGHero or other tools

[coveralls]

Pull Request Test Coverage Report for Build a5486d9f-6237-47d0-8ee4-8d56bd4cafe5

• 4 of 7 (57.14%) changed or added relevant lines in 2 files are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage decreased (-0.02%) to 75.313%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/block_scout_web/lib/block_scout_web/views/api_docs_view.ex	4	5	80.0%
apps/block_scout_web/lib/block_scout_web/views/tokens/instance/overview_view.ex	0	2	0.0%

Files with Coverage Reduction	New Missed Lines	%
apps/block_scout_web/lib/block_scout_web/controllers/chain/market_history_chart_controller.ex	2	71.43%

Totals
Change from base Build 403d0cf5-4417-4757-8b1b-65692cb5992e:	-0.02%
Covered Lines:	5415
Relevant Lines:	7190

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💛 - Coveralls